error: Undefined name 'uploadTask'. Flutter error

Question

I am integrating file uploading system in my flutter project using firebase. While making progress indicator I am getting undefined name 'uploadTask' error.

In this project we can upload file,image and many more to firebase and simultaneously view at the same time.

import 'dart:io';
import 'package:file_picker/file_picker.dart';
import 'package:flutter/material.dart';
import 'package:firebase_core/firebase_core.dart';
import 'package:firebase_storage/firebase_storage.dart';

Future main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await Firebase.initializeApp();
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  PlatformFile? pickedFile;
  UploadTask? uploadTask;
  Future selectFile() async {
    final result = await FilePicker.platform.pickFiles();
    if (result == null) return;

    setState(() {
      pickedFile = result.files.first;
    });
  }

  Future uploadFile() async {
    final path = 'files/${pickedFile!.name}';
    final file = File(pickedFile!.path!);
    final ref = FirebaseStorage.instance.ref().child(path);
    uploadTask = ref.putFile(file);
    final snapshot = await uploadTask!.whenComplete(() => null);
    final urlDownload = await snapshot.ref.getDownloadURL();
    print('Download link: $urlDownload');
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: SafeArea(
          child: Column(
            // mainAxisAlignment: MainAxisAlignment.spaceEvenly,
            children: [
              if (pickedFile != null)
                Expanded(
                  child: Container(
                    color: Colors.blue[100],
                    child: Center(
                      child: Image.file(
                        File(pickedFile!.path!),
                        width: double.infinity,
                        fit: BoxFit.cover,
                      ),
                    ),
                  ),
                ),
              SizedBox(
                height: 32,
              ),
              ElevatedButton(
                onPressed: selectFile,
                child: const Text(
                  'Select File',
                ),
              ),
              ElevatedButton(
                onPressed: uploadFile,
                child: const Text(
                  'Upload File',
                ),
              ),
              buildProgress(),
            ],
          ),
        ),
      ),
    );
  }
}

Widget buildProgress() => StreamBuilder<TaskSnapshot>(
      stream: uploadTask?.snapshotEvents,
      builder: (context, snapshot) {
        if (snapshot.hasData) {
          final data = snapshot.data!;
          double progress = data.bytesTransferred / data.totalBytes;
          return SizedBox(
            height: 50,
            child: Stack(
              fit: StackFit.expand,
              children: [
                LinearProgressIndicator(
                  value: progress,
                  backgroundColor: Colors.grey,
                  color: Colors.green,
                ),
                Center(
                  child: Text(
                    '${(100 * progress).roundToDouble()}%',
                    style: const TextStyle(color: Colors.white),
                  ),
                )
              ],
            ),
          );
        } else {
          return const SizedBox(
            height: 50,
          );
        }
      },
    );