我有一个问题，与空，我不知道为什么..我乞讨的Flutter
“对null调用了方法'map'。接收方：空尝试呼叫：Map（闭合：（字符串）=〉答案）”
main.dart
'

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

void main() => runApp(MyApp());

class MyApp extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    // TODO: implement createState
    return _MyAppState();
  }
}

class _MyAppState extends State<MyApp> {
  static const questions = [
    {
      'questionText': 'Whats your favourite car?',
      'answers': ['HYUNDAI', 'BMW', 'KIA', 'AUDI']
    },
    {
      'questionText': 'What is your favourite game?',
      'answers': ['CS:GO', 'LOL', 'WOW', 'GTA']
    },
    {
      'questionText': 'What is your favourite phone?',
      'answer': ['IPHONE', 'SAMSUNG', 'SONY', 'HUAWEI']
    }
  ];

  var _questionIndex = 0;
  void _answerQuestion() {
    setState(() {
      _questionIndex = _questionIndex + 1;
    });

    if (_questionIndex < questions.length) {
      print('We have more questions');
    }

    print(_questionIndex);
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Scaffold(
          appBar: AppBar(
            backgroundColor: Colors.red,
            title: Text('Quiz App'),
          ),
          body: _questionIndex < questions.length
              ? Column(
                  children: [
                    Question(
                      questions[_questionIndex]['questionText'] as String,
                    ),
                    ...(questions[_questionIndex]['answers'] as List<String>)
                        .map((answer) {
                      return Answer(_answerQuestion, answer);
                    }).toList(),
                  ],
                )
              : Center(child: Text('We finally got it'))),
    );
  }
}

question.dart

import 'package:flutter/material.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: 22),
        textAlign: TextAlign.center,
      ),
    );
  }
}

answer.dart

>

import 'package:flutter/material.dart';

class Answer extends StatelessWidget {
  final VoidCallback selectHandler;
  final String answerText;
  Answer(this.selectHandler, this.answerText);

  @override
  Widget build(BuildContext context) {
    return Container(
        width: double.infinity,
        child: ElevatedButton(
          style: ButtonStyle(
            backgroundColor: MaterialStateProperty.all(Colors.red),
          ),
          onPressed: selectHandler,
          child: Text(answerText),
        ));
  }
}

'
enter image description here
请帮助我：）
我查找了关于此问题的不同主题，将？？添加到toList（），但仍然出现相同的错误