Cant figure how to assign proper values of answers from a List<String> at a multiple choice quiz at flutter,

Question

At listAnswers[1] i did try to assign the string 'white' at the text view but it crashes my program, anyone have any idea why, here is the Class questions which contains a string question, list with answers and a string with correct answer to evaluate, and a quiz class to build the quiz.
And the error log is NoSuchMethodError: The method '[]' was called on null. Reciever: Null Tried Calling: [] (1) NoSuchMethodError: The method '[]' was called on null. Reciever: Null Tried Calling: [] (1)

class _MyHomePageState extends State<MyHomePage> {
Questions currentQuestion;

Quiz quiz = new Quiz([
new Questions(
    "Color of the snow is ", ["yellow", "white", "grey"], "white"),]);
String questionText;
int questionNumber;
String isCorrect;
List<String> listAnswers;

@override
 void initState() {
 super.initState();
 currentQuestion = quiz.nextQuestion;
 questionText = currentQuestion.question;
 questionNumber = quiz.questionNumber;
 listAnswers = quiz.answers;
 isCorrect = quiz.correctAnswer;
}

@override
Widget build(BuildContext context) {
return new Scaffold(
  appBar: new AppBar(
    title: new Text(widget.title),
  ),
  body: new InkWell(
    child: Center(
      child: new Column(
        children: <Widget>[
          Padding(
            padding: const EdgeInsets.all(8.0),
            child: new Text(
              questionText,
              maxLines: questionNumber,
              style: new TextStyle(fontSize: 20.0),
            ),
          ),
          Padding(
            padding: const EdgeInsets.all(8.0),
            child: new RaisedButton(
              // child: new Text(Questions.listAnswers.toString()),
              child: new Text(
                quiz.answers.toString(),
                maxLines: questionNumber,
              ),
              onPressed: _onPressed,
            ),
          ),
          Padding(
            padding: const EdgeInsets.all(8.0),
            child: new Text(
              listAnswers[0],
              style: new TextStyle(fontSize: 20.0),
            ),
          ),
        ],
      ),
    ),
  ),
);
}
}

class Quiz {
List<Questions> _questions;
int _currentQuestionIndex = -1;
int _point = 0;
List<String> _answers;
String _correctAnswer;

Quiz(this._questions) {
_questions.shuffle();
}

List<Questions> get questions => _questions;
List get answers => _answers;
String get correctAnswer => _correctAnswer;
int get length => _questions.length;
int get questionNumber => _currentQuestionIndex + 1;
int get point => _point;

Questions get nextQuestion {
_currentQuestionIndex++;
if (_currentQuestionIndex >= length) return null;
return _questions[_currentQuestionIndex];
} 
}

class Questions {
final String question;
final List<String> answers;
final String correctAnswer;

Questions(this.question, this.answers, this.correctAnswer);
}

Answer 1

Try this.

import 'package:flutter/material.dart';

void main() => runApp(new MaterialApp(
      home: new MainPage(),
      debugShowCheckedModeBanner: false,
    ));

class MainPage extends StatefulWidget {
  @override
  _MainPageState createState() => new _MainPageState();
}

class _MainPageState extends State<MainPage> {
  Questions currentQuestion;

  Quiz quiz = new Quiz([
    new Questions(
        "Color of the snow is ", ["yellow", "white", "grey"], "white"),
  ]);
  String questionText;
  int questionNumber;
  String isCorrect;
  List<String> listAnswers;

  @override
  void initState() {
    super.initState();
    currentQuestion = quiz.nextQuestion;
    questionText = currentQuestion.question;
    questionNumber = quiz.questionNumber;
    listAnswers = quiz.answers;
    isCorrect = quiz.correctAnswer;
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        title: new Text('Quiz'),
      ),
      body: new InkWell(
        child: Center(
          child: new Column(
            children: <Widget>[
              Padding(
                padding: const EdgeInsets.all(8.0),
                child: new Text(
                  questionText,
                  maxLines: questionNumber,
                  style: new TextStyle(fontSize: 20.0),
                ),
              ),
              Padding(
                padding: const EdgeInsets.all(8.0),
                child: buildAnswerButtons(0),
              ),
              Padding(
                padding: const EdgeInsets.all(8.0),
                child: new Text(
                  quiz._questions[0].correctAnswer,
                  style: new TextStyle(fontSize: 20.0),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }

  Widget buildAnswerButtons(int questionPos) {
    List<Widget> buttons = [];
    for (String answer in quiz._questions[questionPos].answers) {
      buttons.add(
        new RaisedButton(
          child: new Text(answer),
          onPressed: () {},
        ),
      );
    }
    return new Row(
      mainAxisSize: MainAxisSize.min,
      children: buttons,
    );
  }
}

class Quiz {
  List<Questions> _questions;
  int _currentQuestionIndex = -1;
  int _point = 0;
  List<String> _answers;
  String _correctAnswer;

  Quiz(this._questions) {
    _questions.shuffle();
  }

  List<Questions> get questions => _questions;

  List get answers => _answers;

  String get correctAnswer => _correctAnswer;

  int get length => _questions.length;

  int get questionNumber => _currentQuestionIndex + 1;

  int get point => _point;

  Questions get nextQuestion {
    _currentQuestionIndex++;
    if (_currentQuestionIndex >= length) return null;
    return _questions[_currentQuestionIndex];
  }
}

class Questions {
  final String question;
  final List<String> answers;
  final String correctAnswer;

  Questions(this.question, this.answers, this.correctAnswer);
}

Answer 2

Your List<String> _answers is not initialized which means it is equal to null. You need to give it an initial value =[] or =new List()