Exception caught by widgets library ,The following assertion was thrown building Answer **error** in flutter

Question

I'm new to flutter , or programming to be clear . I was following a course , doing the same as the Instructor , the app worked for him , but when i try to run it ,this error shows up .

// @dart=2.9

import 'package:flutter/material.dart';
import 'package:flutter/services.dart';
import './question.dart';
import './answer.dart';



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

  Quiz(
      {@required this.questions,
      @required this.answerQuestion,
      @required this.questionIndex});

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

Answer 1

This means that your map has a null value somewhere. Change your code to this:

Answer(() => answerQuestion(answer['score']??"NA"), answer['text']??"NA");

Answer 2

The problem is propably that the person from the course uses an older version of Flutter which doesn't use null safety . So to solve this you need to migrate your code to null safety. For more information on how and when to use null safety you can refer to this great video from Robert Brunehage . The Fixes you need to do on your code are the following:

1. change @required to required

2. add a question Mark to variables that don't have a value,

Bad example:

int questionIndex;

This will lead to a null error because name doesnt have a value (means it is null)

Correct example

int? questionIndex;

This variable is now null safe which prevents it from ending up with an error.

3. after you've now made your variables without a default value null safe you have to add ! at some places (your IDE should show you where to add it normally)

Wrong code:

questions[questionIndex]['questionText'],

Correct code

questions[questionIndex!]['questionText'],

If you don't want to use Null safety (not recommended!)

you can give your Widget a default Text in case your variable has no value (is null)

Question(questions[questionIndex]['questionText']?? "default value",),