我是Flutter的新手。我正在构建一个测验应用程序，并有以下三个dart文件：

主.dart

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

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

class MyApp extends StatefulWidget {
  State<StatefulWidget> createState(){
    return _MyAppState();
  }
}

class _MyAppState extends State<MyApp>{
  var _questionIndex = 0;
  _answerQuestion(){
    setState(() {
      _questionIndex = _questionIndex + 1;
    });
}
  @override
  Widget build(BuildContext context) {
    var questions = [
      {'questionText': 'What\'s your favourite color ?',
        'answers': ['Red','Blue','White','Black']
      },
      {'questionText': 'What\'s your favourite Animal ?',
        'answers': ['Dog','Rabbit','Tiger','Monkey']
      },
      {'questionText': 'What\'s your favourite Day ?',
        'answers': ['Tuesday','Monday','Sunday','Friday','Wednesday','Saturday']
      },
    ];
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('My First App'),
        ),
        body: Column(
          children: [
            Question(questions[_questionIndex]['questionText'] as String,
            ),
            ...(questions[_questionIndex]['answers'] as List).map((answer) {
              return Answer(_answerQuestion(),answer);
            }).toList()
    ],
        )
      ),
    );
  }
}

字符串

**问题. dart **

import 'package:flutter/material.dart';

class Question extends StatelessWidget {
  final String questions;
  Question(this.questions);

  @override
  Widget build(BuildContext context) {
    return Container(
      width: double.infinity,
      margin: EdgeInsets.all(10),
      child:(
      Text(
      questions,
      style: TextStyle(
          fontSize: 25),
      textAlign: TextAlign.center,)
    ),
    );
  }
}

型

**答案. dart **

import 'package:flutter/material.dart';

class Answer  extends StatelessWidget {
  final Function buttonHandler;
  final String answer;

  Answer(this.buttonHandler,this.answer);

  @override
  Widget build(BuildContext context) {
    return Container(
      width: double.infinity,
      child: ElevatedButton(
        child: Text(answer),
        style: ButtonStyle(
            backgroundColor: MaterialStateProperty.all(Colors.blue),
            foregroundColor: MaterialStateProperty.all(Colors.white)
        ),
        onPressed: () => buttonHandler,
    ),
    );
  }
}

型
当我在Android Studio中运行我的Android应用程序时，我得到这个错误：
由WIDGETS LIBRARY提供的电子邮件例外情况
构建MyApp（dirty，state：_MyAppState#7f7de）时抛出以下_TypeError：
类型"函数“不是类型”函数“的子类型
导致错误的相关小部件是：
MyApp文件：/C：/src/first_app/lib/main. dart：7：10