Flutter / Dart / NoSuchMethodError - flutter

Question

I have not been able to fix the error for 2 hours. Maybe one of you can help me. You can find a photo below.
What the Debug Console also tells me is:
[ The following NoSuchMethodError was thrown building MyApp (dirty, state: _MyAppState#c7f7e):
I/flutter (17681): The method 'map' was called on null.
I/flutter (17681): Receiver: null
I/flutter (17681): Tried calling: map<Answer>(Closure: (String) => Answer)]
import 'package:flutter/material.dart';
import './question.dart';
import './answer.dart';
void main() => runApp(MyApp());
class MyApp extends StatefulWidget {
#override
State<StatefulWidget> createState() {
return _MyAppState();
}
}
class _MyAppState extends State<MyApp> {
var _questionIndex = 0;
void _answerQuestion() {
setState(() {
_questionIndex = _questionIndex + 1;
});
print(_questionIndex);
}
#override
Widget build(BuildContext context) {
const questions = const [
{
'questionText': 'What\'s your favorite color?',
'answer': ['Black', 'Yellow', 'white', 'green'],
},
{
'questionText': 'What\'s your favorite animal?',
'answer': ['Lion', 'Rabbit', 'Snake', 'Elephant'],
},
{
'questionText': 'What\'s your favorite instructor?',
'answer': ['Flo', 'Viona', 'Flint', 'Flo'],
},
];
return MaterialApp(
home: Scaffold(
appBar: AppBar(
title: Text('My First App!'),
),
body: Column(
children: [
Question(
questions[_questionIndex]['questionText'],
),
...(questions[_questionIndex]['answers'] as List<String>)
.map((answer) {
return Answer(_answerQuestion, answer);
}).toList()
],
),
),
);
}
}
import 'package:flutter/material.dart';
//import './main.dart';
class Question extends StatelessWidget {
final String questionText;
Question(this.questionText);
#override
Widget build(BuildContext context) {
return Container(
width: double.infinity,
margin: EdgeInsets.all(10),
child: Text(
questionText,
style: TextStyle(fontSize: 28),
textAlign: TextAlign.center,
),
);
}
}
import 'package:flutter/material.dart';
class Answer extends StatelessWidget {
final Function selectHandler;
final String answerText;
Answer(this.selectHandler, this.answerText);
#override
Widget build(BuildContext context) {
return Container(
width: double.infinity,
child: RaisedButton(
color: Colors.blue,
textColor: Colors.white,
child: Text(answerText),
onPressed: selectHandler,
),
);
}
}
That shows me the app

Answer 1

There are no key called answers in your map. Instead it is called answer. So questions[_questionIndex]['answers'] will return null since the [] operator for Map is documented as:
Returns the value for the given key or null if key is not in the map.
https://api.dart.dev/stable/2.8.3/dart-core/Map/operator_get.html

Answer 2

Its due to higher Flutter version you are using. Do the change in pubspec.yaml file. Change the version to 2.11.0
environment:
sdk: ">=2.11.0 <3.0.0"