型別“Null”不是型別轉換錯誤中“String”型別的子型別

我是顫振的初學者，我嘗試做一個基本的測驗型別應用程式，其中包含下面給出的代碼。

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> {
  var questionNo = 0;
  Widget build(BuildContext context) {
    void questionchange() {
      setState(() {
        questionNo = questionNo   1;
      });
      print('Answer 1 is selected');
    }

    const questions = [
      {
        'QuesionText': 'What is your favourite color',
>         'AnswerText': ['Red', 'Blue', 'Violet', 'Green'],
      },
      {
        'QuestionText': 'What is your favourite animal',
        'AnswerText': ['Tiger', 'Lion', 'Elephant', 'Rabbit'],
      },
      {
        'QuesionText': 'What is your favourite Country',
        'AnswerText': ['Rwanda', 'Netherland', 'USA', 'India'],
      }
    ];
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('My first app'),
        ),
        body: Column(
          children: [
            Questions(questions[questionNo]['QuestionText'] as String),
            ...(questions[questionNo]['AnswerText'] as List<String>)
                .map((answer) {
              return Answers(questionchange, answer);
            }).toList(),
          ],
        ),
      ),
    );
  }
}

import 'package:flutter/material.dart';

class Questions extends StatelessWidget {
  final String quest;
  Questions(this.quest);
  @override
  Widget build(BuildContext context) {
    return Container(
        margin: EdgeInsets.all(20),
        width: double.infinity,
        child: Text(
          quest,
          textAlign: TextAlign.center,
          style: TextStyle(fontSize: 26),
        ));
  }
}

import 'package:flutter/material.dart';

class Questions extends StatelessWidget {
  final String quest;
  Questions(this.quest);
  @override
  Widget build(BuildContext context) {
    return Container(
        margin: EdgeInsets.all(20),
        width: double.infinity,
        child: Text(
          quest,
          textAlign: TextAlign.center,
          style: TextStyle(fontSize: 26),
        ));
  }
}

當我嘗試運行它時，應用程式螢屏正在顯示

type 'Null' is not a subtype of type 'String' in type cast在紅色背景。

此外，除錯控制臺顯示為引起 MyApp 小部件和行的相關錯誤Questions(questions[questionNo]['QuestionText'] as String)。

誰能告訴我是什么導致了這個錯誤。

uj5u.com熱心網友回復：

您的輸入有誤：

const questions = [
      {
        'QuesionText': 'What is your favourite color',
        'AnswerText': ['Red', 'Blue', 'Violet', 'Green'],
      },
      {
        'QuestionText': 'What is your favourite animal',
        'AnswerText': ['Tiger', 'Lion', 'Elephant', 'Rabbit'],
      },
      {
        'QuesionText': 'What is your favourite Country',
        'AnswerText': ['Rwanda', 'Netherland', 'USA', 'India'],
      }
    ];

因為你寫的是“QuestionText”而不是“QuestionText”，它應該是：

   const questions = [
      {
        'QuestionText': 'What is your favourite color',
        'AnswerText': ['Red', 'Blue', 'Violet', 'Green'],
      },
      {
        'QuestionText': 'What is your favourite animal',
        'AnswerText': ['Tiger', 'Lion', 'Elephant', 'Rabbit'],
      },
      {
        'QuestionText': 'What is your favourite Country',
        'AnswerText': ['Rwanda', 'Netherland', 'USA', 'India'],
      }
    ];

那是因為當你在這里閱讀它時：

   Questions(questions[questionNo]['QuestionText'] as String),

它會給你一個空錯誤。

uj5u.com熱心網友回復：

您的兩把鑰匙有錯字；替換'QuesionText'為'QuestionText'.

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