Exception: Context is null, please call ToastContext.init(context) first

Question

The issue is seen on the Toast.dart and Armodel.dart. if possible please suggest me how do i implement OCR as i followed up the codes from Pubdev but still whenever i opens my camera it crashes.

import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

class ToastContext {
  BuildContext? context;
  MethodChannel? _channel;

  static final ToastContext _instance = ToastContext._internal();

  /// Prmary Constructor for FToast
  factory ToastContext() {
    return _instance;
  }

  /// Take users Context and saves to avariable
  ToastContext init(BuildContext context) {
    _instance.context = context;
    return _instance;
  }

  ToastContext._internal();
}

class Toast {
  static const int lengthShort = 1;
  static const int lengthLong = 3;
  static const int bottom = 0;
  static const int center = 1;
  static const int top = 2;
  static void show(String msg,
      {int? duration = 1,
      int? gravity = 0,
      Color backgroundColor = const Color(0xAA000000),
      textStyle = const TextStyle(fontSize: 15, color: Colors.white),
      double backgroundRadius = 20,
      bool? rootNavigator,
      Border? border,
      bool webShowClose = false,
      Color webTexColor = const Color(0xFFffffff)}) {
    if (ToastContext().context == null) {
      throw Exception('Context is null, please call ToastContext.init(context) first');
    }
    if (kIsWeb == true) {
      if (ToastContext()._channel == null) {
        ToastContext()._channel = const MethodChannel('appdev/FlutterToast');
      }
      String toastGravity = "bottom";
      if (gravity == Toast.top) {
        toastGravity = "top";
      } else if (gravity == Toast.center) {
        toastGravity = "center";
      } else {
        toastGravity = "bottom";
      }

      final Map<String, dynamic> params = <String, dynamic>{
        'msg': msg,
        'duration': (duration ?? 1) * 1000,
        'gravity': toastGravity,
        'bgcolor': backgroundColor.toString(),
        'textcolor': webTexColor.value.toRadixString(16),
        'webShowClose': webShowClose,
      };
      ToastContext()._channel?.invokeMethod("showToast", params);
    } else {
      ToastView.dismiss();
      ToastView.createView(msg, ToastContext().context!, duration, gravity, backgroundColor,
          textStyle, backgroundRadius, border, rootNavigator);
    }
  }
}

class ToastView {
  static final ToastView _singleton = ToastView._internal();

  factory ToastView() {
    return _singleton;
  }

  ToastView._internal();

  static OverlayState? overlayState;
  static OverlayEntry? _overlayEntry;
  static bool _isVisible = false;

  static void createView(
      String msg,
      BuildContext context,
      int? duration,
      int? gravity,
      Color background,
      TextStyle textStyle,
      double backgroundRadius,
      Border? border,
      bool? rootNavigator) async {
    overlayState = Overlay.of(context, rootOverlay: rootNavigator ?? false);

    _overlayEntry = OverlayEntry(
      builder: (BuildContext context) => ToastWidget(
          widget: SizedBox(
            width: MediaQuery.of(context).size.width,
            child: Container(
                alignment: Alignment.center,
                width: MediaQuery.of(context).size.width,
                child: Container(
                  decoration: BoxDecoration(
                    color: background,
                    borderRadius: BorderRadius.circular(backgroundRadius),
                    border: border,
                  ),
                  margin: const EdgeInsets.symmetric(horizontal: 20),
                  padding: const EdgeInsets.fromLTRB(16, 10, 16, 10),
                  child: Text(msg, softWrap: true, style: textStyle),
                )),
          ),
          gravity: gravity),
    );
    _isVisible = true;
    overlayState!.insert(_overlayEntry!);
    await Future.delayed(Duration(seconds: duration ?? Toast.lengthShort));
    dismiss();
  }

  static dismiss() async {
    if (!_isVisible) {
      return;
    }
    _isVisible = false;
    _overlayEntry?.remove();
  }
}

class ToastWidget extends StatelessWidget {
  const ToastWidget({
    Key? key,
    required this.widget,
    required this.gravity,
  }) : super(key: key);

  final Widget widget;
  final int? gravity;

  @override
  Widget build(BuildContext context) {
    return Positioned(
        top: gravity == 2 ? MediaQuery.of(context).viewInsets.top + 50 : null,
        bottom: gravity == 0 ? MediaQuery.of(context).viewInsets.bottom + 50 : null,
        child: IgnorePointer(
          child: Material(
            color: Colors.transparent,
            child: widget,
          ),
        ));
  }
}

import 'package:firebase_database/firebase_database.dart';
import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:flutter_mobile_vision_2/flutter_mobile_vision_2.dart';
import 'package:toast/toast.dart';

import 'arviewpage.dart';

class Armodelselect extends StatefulWidget {
  @override
  _ArmodelselectState createState() => _ArmodelselectState();
}

class _ArmodelselectState extends State<Armodelselect> {
  int _cameraOcr = 0;
  //String _textValue = "sample";
  String position = "";
  bool vision = false;
  bool showwords = true;
  bool search = false;
  late String searchtext;
  bool flash = false;
  List<OcrText> scannedwords = [];
  var DATA, DATA2, DATA3;
  final databasepost = FirebaseDatabase.instance.reference();
  final databasepost1 = FirebaseDatabase.instance.reference();
  late OcrText selected;

  @override
  void initState() {
    getdata();
    FlutterMobileVision.start().then((x) => setState(() {
          vision = true;
        }));
    super.initState();
  }

  Future<List<OcrText>> _getwords() async {
    return scannedwords;
  }

  Future<bool> _onWillPop() async {
    SystemNavigator.pop();
    return false;
  }

  @override
  Widget build(BuildContext context) {
    return WillPopScope(
      onWillPop: _onWillPop,
      child: Container(
        height: MediaQuery.of(context).size.height - 185.0,
        decoration: BoxDecoration(
          color: Colors.white,
          borderRadius: BorderRadius.only(topLeft: Radius.circular(75.0)),
        ),
        child: NestedScrollView(
          headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
            return <Widget>[
              SliverAppBar(
                backgroundColor: Colors.transparent,
                expandedHeight: 100,
                floating: false,
                pinned: false,
                automaticallyImplyLeading: false,
                flexibleSpace: FlexibleSpaceBar(
                  background:
                      Row(mainAxisAlignment: MainAxisAlignment.end, children: <
                          Widget>[
                    Padding(
                      padding: const EdgeInsets.all(10.0),
                      child: Container(
                        decoration: BoxDecoration(
                          color: Colors.grey[100],
                          border: Border.all(color: Colors.blue, width: 1.0),
                          borderRadius: BorderRadius.all(Radius.circular(5.0)),
                          boxShadow: [
                            BoxShadow(
                                blurRadius: 5,
                                color: Colors.black54,
                                offset: Offset(1, 3))
                          ],
                        ),
                        child: IconButton(
                          icon: Icon(
                            Icons.refresh,
                            color: Colors.blue[500],
                          ),
                          onPressed: () {
                            setState(() {
                              getdata();
                            });
                          },
                        ),
                      ),
                    ),
                    Visibility(
                      visible: !search,
                      child: Padding(
                        padding: const EdgeInsets.all(10.0),
                        child: Container(
                          decoration: BoxDecoration(
                            color: Colors.grey[100],
                            border: Border.all(color: Colors.blue, width: 1.0),
                            borderRadius:
                                BorderRadius.all(Radius.circular(5.0)),
                            boxShadow: [
                              BoxShadow(
                                  blurRadius: 5,
                                  color: Colors.black54,
                                  offset: Offset(1, 3))
                            ],
                          ),
                          child: IconButton(
                            icon: Icon(
                              Icons.camera_alt,
                              color: Colors.blue[500],
                            ),
                            onPressed: () {
                              camerarefresh();
                            },
                          ),
                        ),
                      ),
                    ),
                    Visibility(
                      visible: !search,
                      child: Padding(
                        padding: const EdgeInsets.all(10.0),
                        child: Container(
                          decoration: BoxDecoration(
                            color: Colors.grey[100],
                            border: Border.all(color: Colors.blue, width: 1.0),
                            borderRadius:
                                BorderRadius.all(Radius.circular(5.0)),
                            boxShadow: [
                              BoxShadow(
                                  blurRadius: 5,
                                  color: Colors.black54,
                                  offset: Offset(1, 3))
                            ],
                          ),
                          child: IconButton(
                            icon:
                                Icon(flash ? Icons.flash_on : Icons.flash_off),
                            color: Colors.blue,
                            onPressed: () {
                              setState(() {
                                if (flash) {
                                  flash = false;
                                } else {
                                  flash = true;
                                }
                              });
                            },
                          ),
                        ),
                      ),
                    ),
                    Visibility(
                      visible: !search,
                      child: Padding(
                        padding: const EdgeInsets.all(10.0),
                        child: Container(
                          decoration: BoxDecoration(
                            color: Colors.grey[100],
                            border: Border.all(color: Colors.blue, width: 1.0),
                            borderRadius:
                                BorderRadius.all(Radius.circular(5.0)),
                            boxShadow: [
                              BoxShadow(
                                  blurRadius: 5,
                                  color: Colors.black54,
                                  offset: Offset(1, 3))
                            ],
                          ),
                          child: IconButton(
                            icon: Icon(
                              Icons.list,
                              color: Colors.blue[500],
                            ),
                            onPressed: () {
                              setState(() {
                                if (showwords) {
                                  showwords = false;
                                } else {
                                  showwords = true;
                                }
                              });
                            },
                          ),
                        ),
                      ),
                    ),
                    Padding(
                      padding: const EdgeInsets.all(10.0),
                      child: Container(
                        decoration: BoxDecoration(
                          color: Colors.grey[100],
                          border: Border.all(color: Colors.blue, width: 1.0),
                          borderRadius: BorderRadius.all(Radius.circular(5.0)),
                          boxShadow: [
                            BoxShadow(
                                blurRadius: 5,
                                color: Colors.black54,
                                offset: Offset(1, 3))
                          ],
                        ),
                        child: IconButton(
                          icon: Icon(
                            Icons.search,
                            color: Colors.blue[500],
                          ),
                          onPressed: () {
                            setState(() {
                              if (search) {
                                search = false;
                              } else {
                                search = true;
                              }
                            });
                          },
                        ),
                      ),
                    ),
                    Visibility(
                      visible: search,
                      child: Expanded(
                        child: Padding(
                          padding: const EdgeInsets.all(10.0),
                          child: TextField(
                              decoration: InputDecoration(
                                  labelText: "Keyword",
                                  border: OutlineInputBorder()),
                              onChanged: (text) {
                                setState(() {
                                  searchtext = text;
                                });
                              }),
                        ),
                      ),
                    ),
                  ]),
                ),
              )
            ];
          },
          body: Container(
            padding: EdgeInsets.fromLTRB(50, 10, 0, 0),
            child: FutureBuilder(
                future: _getwords(),
                builder: (BuildContext context, AsyncSnapshot snapshot) {
                  if (search) {
                    return Center(
                      child: RaisedButton(
                        color: Color(0xFF21BFBD),
                        onPressed: () {
                          searchbutton(searchtext);
                        },
                        child: Text("Build Model"),
                      ),
                    );
                  } else if (scannedwords.length == 0 || !showwords) {
                    return Container();
                  } else if (scannedwords.length == 1) {
                    wordselected(scannedwords[0]);
                    return Container();
                  } else {
                    return Container(
                      child: ListView.builder(
                          scrollDirection: Axis.vertical,
                          shrinkWrap: true,
                          itemCount: scannedwords.length,
                          itemBuilder: (BuildContext context, int index) {
                            return Padding(
                              padding: const EdgeInsets.all(8.0),
                              child: GestureDetector(
                                onTap: () {
                                  wordselected(scannedwords[index]);
                                },
                                child: Container(
                                  decoration: BoxDecoration(
                                    color: Colors.grey[100],
                                    border: Border.all(
                                        color: Colors.blue, width: 2.0),
                                    borderRadius:
                                        BorderRadius.all(Radius.circular(5.0)),
                                    boxShadow: [
                                      BoxShadow(
                                          blurRadius: 5,
                                          color: Colors.black54,
                                          offset: Offset(1, 3))
                                    ],
                                  ),
                                  padding: EdgeInsets.all(10),
                                  child: Center(
                                    child: Text(
                                      processword(
                                        scannedwords[index].value.toString(),
                                      ),
                                      style: TextStyle(
                                        color: Colors.black,
                                        fontWeight: FontWeight.w400,
                                        fontSize: 30,
                                      ),
                                    ),
                                  ),
                                ),
                              ),
                            );
                          }),
                    );
                  }
                }),
          ),
        ),
      ),
    );
  }

  //functions:::::::::::::::
  Future<void> camerarefresh() async {
    _read();
  }

  //read live words from camera
  _read() async {
    try {
      print(" $_cameraOcr");
      scannedwords = await FlutterMobileVision.read(
        camera: 0,
        flash: flash,
        waitTap: true,
        autoFocus: true,
        multiple: true,
        showText: true,
        fps: 30,
      );

      Future.delayed(Duration(seconds: 1), () {
        setState(() {});
      });
    } catch (e) {
      showToast("failed to recognize text");
    }
  }

  //method when ocr word list is shown
  Future<void> wordselected(OcrText word) async {
    String name = word.value.toString().toLowerCase();
    if (!checkname(name, DATA2['available'])) {
      showToast("Ar data not available check after sometime");
      changerequestword(name, DATA2['requested']);
    } else {
      print("${DATA[name]['image']}");
      objmodel modeldes = new objmodel(
          name,
          DATA[name]['size'].toString().split(","),
          DATA[name]['image'],
          DATA[name]['explain'],
          DATA[name]['type']);
      Future.delayed(Duration(seconds: 2), () {
        vision = false;
        Navigator.push(
          context,
          MaterialPageRoute(
            builder: (context) => arviewpage(modeldes),
          ),
        );
      });
    }
  }

  //for search keyword button method implementation
  Future<void> searchbutton(String word) async {
    String name = word.toLowerCase();
    print(DATA[0]);
    if (!checkname(name, DATA2['available'])) {
      showToast("Ar data not available check after sometime");
      changerequestword(name, DATA2['requested']);
    } else {
      print("image${DATA[name]['image']}");
      objmodel modeldes = new objmodel(
          name,
          DATA[name]['size'].toString().split(","),
          DATA[name]['image'],
          DATA[name]['explain'],
          DATA[name]['type']);
      Future.delayed(Duration(seconds: 2), () {
        vision = false;
        Navigator.push(
          context,
          MaterialPageRoute(
            builder: (context) => arviewpage(modeldes),
          ),
        );
      });
    }
  }

  //check if the word exist in the available,requested word list
  bool checkname(String word, String list) {
    var listc = list.split(",");
    for (int i = 0; i < listc.length; i++) {
      if (listc[i] == word) {
        return true;
      }
    }
    return false;
  }

  //make wordfrequency change request to firebase database
  changerequestword(String word, String list) {
    if (checkname(word, list)) {
      int count = int.parse(DATA3[word]);
      count = count + 1;
      databasepost.child("wordfrequency").update({
        "$word": "$count",
      });
    } else {
      databasepost.child("wordfrequency").update({
        "$word": "1",
      });
      if (list == "") {
        databasepost1.child("wordlist").update({
          "requested": "${word}",
        });
      } else {
        databasepost1.child("wordlist").update({
          "requested": "${list},${word}",
        });
      }
    }
  }

  String processword(String word) {
    String postword = "";
    word.runes.forEach((int rune) {
      var character = new String.fromCharCode(rune);
      if (character == " ") {
        postword = postword + " ";
      } else {
        postword = postword + character;
      }
    });
    var w = postword.split("\n");
    postword = "";
    for (int i = 0; i < w.length; i++) {
      postword = postword + w[i] + " ";
    }
    return postword.trim();
  }

  //toast method
  void showToast(String text) {
    Toast.show(text,
        textStyle: context, duration: Toast.lengthLong, gravity: Toast.bottom);
  }

  //get url for image from database
  Future<void> getdata() async {
    final reference =
        await FirebaseDatabase.instance.reference().child("datamodel");
    reference.once().then((DatabaseEvent snap) {
      DATA = snap.snapshot;
    });
    final reference2 =
        await FirebaseDatabase.instance.reference().child("wordlist");
    reference2.once().then((DatabaseEvent snap) {
      DATA2 = snap.snapshot;
    });
    final reference3 =
        await FirebaseDatabase.instance.reference().child("wordfrequency");
    reference3.once().then((DatabaseEvent snap) {
      DATA3 = snap.snapshot;
    });
  }
}

class objmodel {
  var size;
  String name, img_url, type, explain;
  objmodel(this.name, this.size, this.img_url, this.explain, this.type);
}

E/flutter ( 6018): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: Exception: Context is null, please call ToastContext.init(context) first
E/flutter ( 6018): #0      Toast.show (package:toast/toast.dart:42:7)
E/flutter ( 6018): #1      _ArmodelselectState.showToast (package:flutter_auth_ui/screens/armodelselect.dart:456:11)
E/flutter ( 6018): #2      _ArmodelselectState._read (package:flutter_auth_ui/screens/armodelselect.dart:344:7)
E

Answer 1

I think you should call this code in your main function.

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    ToastContext().init(context);//-> This part
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: new MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

Answer 2

Because in your Toast class, you have:

if (ToastContext().context == null) {
      throw Exception('Context is null, please call ToastContext.init(context) first');
}

So it is obvious that if your context is null, there will be an exception thrown. And in the code you have posted, there is no any .init(...) method called to set the context of ToastContext class?

You have to call init method of the ToastContext class wherever you are initializing the page. Better to call it in the initState, or wherever BUT before using it!