Failed assertion: boolean expression must not be null ?? How to fix this?

Question

when I trying to run this app I got the following error.

Failed assertion: boolean expression must not be null
    The relevant error-causing widget was: 
      MyApp file:///E:/Flutter/quizmakerapp/lib/main.dart:9:10
    When the exception was thrown, this was the stack: 
    #0      _MyAppState.build (package:quizapp2/main.dart:44:13)
    #1      StatefulElement.build (package:flutter/src/widgets/framework.dart:4663:28)
    #2      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:4546:15)
    #3      StatefulElement.performRebuild (package:flutter/src/widgets/framework.dart:4719:11)
    #4      Element.rebuild (package:flutter/src/widgets/framework.dart:4262:5)

Hi, I am making a Quiz App using a flutter and android studio. when I implementing this app I got an error like this. those are my files when I'm trying to run this it gives the above error. I tried some solutions but it didn't work for me. and also I am new to flutter. Any ideas on the issue?

main.dart

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import 'package:quizapp2/helper/authenticate.dart';
import 'package:quizapp2/helper/constants.dart';
import 'package:quizapp2/views/home.dart';
import 'package:shared_preferences/shared_preferences.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatefulWidget {
  // This widget is the root of your application.
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  bool isUserLoggedIn = false;

  @override
  void initState() {
    getLoggedInState();
    super.initState();
  }

  getLoggedInState() async {
    await Constants.getUerLoggedInSharedPreference().then((value) {
      setState(() {
        isUserLoggedIn = value;
      });
    });
  }

  @override
  Widget build(BuildContext context) {

    return MaterialApp(
      title: 'Quiz App',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: isUserLoggedIn ? Home() : Authenticate(),
    );
  }
}

home.dart

import 'package:flutter/material.dart';
import 'package:quizapp2/services/database.dart';
import 'package:quizapp2/views/create_quiz.dart';
import 'package:quizapp2/views/quiz_play.dart';
import 'package:quizapp2/widget/widget.dart';

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  Stream quizStream;
  DatabaseService databaseService = new DatabaseService();

  Widget quizList() {
    return Container(
      child: Column(
        children: [
          StreamBuilder(
            stream: quizStream,
            builder: (context, snapshot) {
              return snapshot.data == null
                  ? Container()
                  : ListView.builder(
                      shrinkWrap: true,
                      physics: ClampingScrollPhysics(),
                      itemCount: snapshot.data.documents.length,
                      itemBuilder: (context, index) {
                        return QuizTile(
                          noOfQuestions: snapshot.data.documents.length,
                          imageUrl:
                              snapshot.data.documents[index].data['quizImgUrl'],
                          title:
                              snapshot.data.documents[index].data['quizTitle'],
                          description:
                              snapshot.data.documents[index].data['quizDesc'],
                          id: snapshot.data.documents[index].data["id"],
                        );
                      });
            },
          )
        ],
      ),
    );
  }

  @override
  void initState() {
    databaseService.getQuizData().then((value) {
      quizStream = value;
      setState(() {});
    });
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Colors.white,
      appBar: AppBar(
        title: AppLogo(),
        brightness: Brightness.light,
        elevation: 0.0,
        backgroundColor: Colors.transparent,
        //brightness: Brightness.li,
      ),
      body: quizList(),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add),
        onPressed: () {
          Navigator.push(
              context, MaterialPageRoute(builder: (context) => CreateQuiz()));
        },
      ),
    );
  }
}

class QuizTile extends StatelessWidget {
  final String imageUrl, title, id, description;
  final int noOfQuestions;

  QuizTile(
      {@required this.title,
      @required this.imageUrl,
      @required this.description,
      @required this.id,
      @required this.noOfQuestions});

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: (){
        Navigator.push(context, MaterialPageRoute(
          builder: (context) => QuizPlay(id)
        ));
      },
      child: Container(
        padding: EdgeInsets.symmetric(horizontal: 24),
        height: 150,
        child: ClipRRect(
          borderRadius: BorderRadius.circular(8),
          child: Stack(
            children: [
              Image.network(
                imageUrl,
                fit: BoxFit.cover,
                width: MediaQuery.of(context).size.width,
              ),
              Container(
                color: Colors.black26,
                child: Center(
                  child: Column(
                    mainAxisAlignment: MainAxisAlignment.center,
                    children: [
                      Text(
                        title,
                        style: TextStyle(
                            fontSize: 18,
                            color: Colors.white,
                            fontWeight: FontWeight.w500),
                      ),
                      SizedBox(height: 4,),
                      Text(
                        description,
                        style: TextStyle(
                            fontSize: 13,
                            color: Colors.white,
                            fontWeight: FontWeight.w500),
                      )
                    ],
                  ),
                ),
              )
            ],
          ),
        ),
      ),
    );
  }
}

authenticate.dart

import 'package:flutter/material.dart';
import 'package:quizapp2/views/signin.dart';
import 'package:quizapp2/views/signup.dart';

class Authenticate extends StatefulWidget {
  @override
  _AuthenticateState createState() => _AuthenticateState();
}

class _AuthenticateState extends State<Authenticate> {
  bool showSignIn = true;

  void toggleView() {
    setState(() {
      showSignIn = !showSignIn;
    });
  }

  @override
  Widget build(BuildContext context) {
    if (showSignIn) {
      return SignIn(toogleView: toggleView);
    } else {
      return SignUp(toogleView: toggleView);
    }
  }
}

Could you please help me on how to fix that error?

Please post the code for `Constants.getUerLoggedInSharedPreference()`. Seems like it might be returning null. — wxker, Sep 01 '20 at 07:37
Thank you very much, `getLoggedInState() async { await Constants.getUerLoggedInSharedPreference().then((value) { setState(() { isUserLoggedIn = value ?? false; }); }); }` I added the `false` — Randika Madushan, Sep 01 '20 at 08:06

score 2 · Accepted Answer · answered Sep 01 '20 at 07:36

You get this when you try to use a null as a boolean. Here it seems getUerLoggedInSharedPreference()'s future returns null.

Either fix that so it always returns either true or false (we don't have the code here to fix)

Or do a fix like this:

    await Constants.getUerLoggedInSharedPreference().then((value) {
      setState(() {
        isUserLoggedIn = value ?? false;
      });
    });

Failed assertion: boolean expression must not be null ?? How to fix this?

1 Answers1