Flutter _CastError (type 'Null' is not a subtype of type 'List<Map<String, dynamic>?>' in type cast)

Question

This is the flutter code that I wrote 2 years ago. But I couldn't make any sense of why it has stopped working now.

How do I fix this exception.

import './question.dart';
import './answer.dart';

class Quiz extends StatelessWidget {
  final List<Map<String, dynamic>> questions;
  final int questionIndex;
  final Function answerQuestion;

  const Quiz({
    super.key,
    required this.questions,
    required this.answerQuestion,
    required this.questionIndex,
  });

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Question(
          questions[questionIndex]['questionText'] as String,
        ),
        ...(questions[questionIndex]['answers'] as List<Map<String, dynamic>?>)
            .map((answer) {
          return Answer(
              () => answerQuestion(answer['score']), answer!['text'] as String);
        }).toList()
      ],
    );
  }
}

This is the GitHub: github.com/Ankitkj1999/gfg_quiz/blob/master/lib/quiz.dart — Ankit Kumar
– Ankit Kumar, Commented Dec 3, 2022 at 13:57
could you include your print("data = ${questions[questionIndex]}") result? — eamirho3ein
– eamirho3ein, Commented Dec 3, 2022 at 14:10
You do know that Dart has null safety now? Two years ago it didn't. — Roslan Amir
– Roslan Amir, Commented Dec 3, 2022 at 14:18

eamirho3ein · Accepted Answer · 2022-12-03 14:22:53Z

1

Change this:

...(questions[questionIndex]['answers'] as List<Map<String, dynamic>?>)

to this:

...(questions[questionIndex]['answers'] as List<Map<String, dynamic>?>?)

this happened because questions[questionIndex]['answers'] is null and can't cast null to List<Map<String, dynamic>?>.

answered Dec 3, 2022 at 14:22

eamirho3ein

18.1k2 gold badges17 silver badges29 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

Ankit Kumar Over a year ago

I had to do this to solve other exceptions. Now it doesn't show any error but the widget is not getting rendered in the screen. ...?(questions[questionIndex]['answers'] as List<Map<String, dynamic>?>?) ?.map((answer) { return Answer( () => answerQuestion(answer['score']), answer!['text'] as String); }).toList()

eamirho3ein Over a year ago

could you include your print("data = ${questions[questionIndex]}") result? I think the data you looking for is empty and that is why you widget doesn't show up.

Ankit Kumar Over a year ago

yes it is null, but we have initialized question in the code.

DemianSixth · Accepted Answer · 2023-04-29 01:01:22Z

1

I cloned your repo from GitHub, replaced quiz.dart with your code from the topic. There are things that must be done since some widgets are outdated:

Change RaisedButton to ElevatedButton (with the removal of color and textColor properties)
Change FlatButton to TextButton (with the removal of textColor property)

Other changes to pubspec.yaml:

environment:
  sdk: ">=2.19.4 <3.0.0"

instead of sdk: ">=2.7.0 <3.0.0"

in result.dart change Function to VoidCallback:

  final VoidCallback resetHandler;

in answers.dart change Function to VoidCallback:

  final VoidCallback selectHandler;

And now, the code you provided here executed normally.

answered Apr 29, 2023 at 1:01

DemianSixth

112 bronze badges

Collectives™ on Stack Overflow

Flutter _CastError (type 'Null' is not a subtype of type 'List<Map<String, dynamic>?>' in type cast)

2 Answers 2

3 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

3 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related