Error is "The getter 'length' was called on null. Receiver: null Tried calling: length"

Question

I am trying to develop a flutter app. This app is like a dating app. After writing all the code, everything is working fine as I expected But on the home screen, I got an exception, where I know where the exception and what it means, but i do not have any clue how i could solve the problem. I hope some of you guys could help me.

This is the code where the error is thrown:

import 'package:buis_talk/blocs/swipe/swipe_bloc.dart';
import 'package:buis_talk/widgets/widgets.dart';
import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:buis_talk/models/models.dart';

class HomeScreen extends StatelessWidget {
  static const String routeName = '/home';

  // const HomeScreen({Key? key}) : super(key: key);

  static Route route() {
    return MaterialPageRoute(
      settings: const RouteSettings(name: routeName),
      builder: (context) => HomeScreen(),
    );
  }

@override
  Widget build(BuildContext context) {
  return Scaffold(
    appBar: const CustomAppBar(
      title: 'Home',
    ),
    body: BlocBuilder<SwipeBloc, SwipeState>(
      builder: (context, state) {
        var userCount = state.users.length;
          return Column(
            children: [
              InkWell(
                onDoubleTap: () {
                  Navigator.pushNamed(context, '/users',
                    arguments: state.users[0]
                    );
                },
                child: Draggable<User>(
                data: state.users[0],
                  child: UserCard(user: state.users[0]),
                  feedback: UserCard(user: state.users[0]),
                  childWhenDragging: (userCount > 1) ?
                    UserCard(user: state.users[1]): Container(),
                  onDragEnd: (drag) {
                    if (drag.velocity.pixelsPerSecond.dx < 0) {
                      context.read<SwipeBloc>()
                        .add(SwipeLeft(user: state.users[0]));
                      print('Swiped left');
                    } else {
                      context.read<SwipeBloc>()
                        .add(SwipeRight(user: state.users[0]));
                      print('Swiped right');
                    }
                  },
                ),
              ),
              Padding(
                padding: const EdgeInsets.symmetric(
                  vertical: 8.0,
                  horizontal: 60,
                ),
                child: Row(
                  mainAxisAlignment: MainAxisAlignment.spaceBetween,
                  children: [
                    InkWell(
                      onTap: () {
                        context.read<SwipeBloc>()
                      .add(SwipeLeft(user: state.users[0]));
                    print('Swiped left');
                      },
                      child: ChoiceButton(
                        width: 60,
                        height: 60,
                        size: 25,
                        hasGradient: false,
                        color: Theme.of(context).colorScheme.secondary,
                        icon: Icons.clear_rounded,
                      ),
                    ),
                    InkWell(
                      onTap: () {
                        context.read<SwipeBloc>()
                      .add(SwipeRight(user: state.users[0]));
                    print('Swiped right');
                      },
                      child: const ChoiceButton(
                        width: 80,
                        height: 80,
                        size: 30,
                        hasGradient: true,
                        color: Colors.white,
                        icon: Icons.coffee,
                      ),
                    ),
                    ChoiceButton(
                      width: 60,
                      height: 60,
                      size: 25,
                      hasGradient: false,
                      color: Theme.of(context).primaryColor,
                      icon: Icons.watch_later,
                    ),
                  ],
                ),
              ),
            ],
          ); 
        }
      ),
    );
  }
}

And the error message is:

The getter 'length' was called on null.
Receiver: null
Tried calling: length

The relevant error-causing widget was:
  BlocBuilder<SwipeBloc, SwipeState>

If I remove length from var usercount = state.users.length; => var usercount = state.users; then it gives me error is:

The method '[]' was called on null.
Receiver: null
Tried calling: [](0)

The relevant error-causing widget was:
  BlocBuilder<SwipeBloc, SwipeState> 

Answer 1

Do this,

var usercount = (state.users ?? []).length;

and its good to have checks, before doing this state.users[0]

check if the list has values like

if(state.users.isNotEmpty){
  state.users[0];
}