dart - フラッターで選択されたトラックラジオボタン

Question

私はクイズアプリ用に以下のコードを書きましたが、選択したラジオボタンを正しい答えと比較する方法を見つけることに行き詰まっています。switch ステートメントと _counter 変数を使用しようとしましたが、エラーになります

The following NoSuchMethodError was thrown while handling a gesture:
I/flutter (28574): The method '[]' was called on null.

私は、エラーがどこ/何を参照しているのか、またはそのアプローチの何が問題なのかを理解するのに十分な知識がありません(switchステートメント)。修正/指示/ヒントをいただければ幸いです。ありがとう。

import 'dart:async';
import 'package:flutter/material.dart';

Map<String, Map<String, String>> questionBank = {
  "1": {
    "question": "What is the capital of Canada?",
    "ans1": "Toronto",
    "ans2": "Montreal",
    "ans3": "Ottawa",
    "ans4": "Vancouver",
    "coAns": "Ottawa"
  },
  "2": {
    "question": "What is the capital of the United States of America?",
    "ans1": "New York",
    "ans2": "California",
    "ans3": "Texas",
    "ans4": "Washington DC",
    "coAns": "Washington DC"
  },
  "3": {
    "question": "What is the capital of Nigeria?",
    "ans1": "Abuja",
    "ans2": "Lagos",
    "ans3": "Port Harcourt",
    "ans4": "Makurdi",
    "coAns": "Abuja"
  },
  "4": {
    "question": "What is the capital of England?",
    "ans1": "Britain",
    "ans2": "Scotland",
    "ans3": "London",
    "ans4": "Edinburgh",
    "coAns": "London"
  },
  "5": {
    "question": "What is the capital of China?",
    "ans1": "Beijing",
    "ans2": "Shanghai",
    "ans3": "Tianjin",
    "ans4": "Taiwan",
    "coAns": "Beijing"
  },
};

void main() {
  runApp(new _questionDisplay());
}

class _questionDisplay extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(home: new QuestDis());
  }
}

class QuestDis extends StatefulWidget {
  QuestDis({Key key}) : super(key: key);

  @override
  _QuestDisState createState() => new _QuestDisState();
}

class _QuestDisState extends State<QuestDis> {

  @override
  var _counter = 1;
  var bkgrdColor = Colors.blue[50];
  int radioValue = 0;

  int ans1Value = 1;
  int ans2Value = 2;
  int ans3Value = 3;
  int ans4Value = 4;

  void handleRadioValueChanged(int value) {
    setState(() {
      radioValue = value;
      /*
      switch (radioValue) {
        case 1:
          bkgrdColor = (questionBank[_counter]["coAns"] ==
                  questionBank[_counter][ans1Value])
              ? Colors.green[50]
              : Colors.red[50];
          break;
        case 2:
          bkgrdColor = (questionBank[_counter]["coAns"] ==
                  questionBank[_counter][ans2Value])
              ? Colors.green[50]
              : Colors.red[50];
          break;
        case 3:
          bkgrdColor = (questionBank[_counter]["coAns"] ==
                  questionBank[_counter][ans3Value])
              ? Colors.green[50]
              : Colors.red[50];
          break;
        case 4:
          bkgrdColor = (questionBank[_counter]["coAns"] ==
                  questionBank[_counter][ans4Value])
              ? Colors.green[50]
              : Colors.red[50];
          break;
      }
       */
      _counter++;
      radioValue = 0;
    });
  }

  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: new AppBar(
        leading: new IconButton(icon: new Icon(Icons.menu), onPressed: null),
        title: new Text('quizApp'),
      ),
      body: new Container(
        child: new Column(
          children: [
            new Expanded(
              child: new Container(
                child: new Column(
                  children: [
                    new Expanded(
                      child: new Container(
                        child: new Card(
                          color: bkgrdColor,
                          child: new Row(
                            children: <Widget>[
                              new Text(
                                  "${questionBank[_counter.toString()]["question"]}"),
                            ],
                          ),
                        ),
                      ),
                    ),
                    new Expanded(
                      child: new Container(
                        child: new Card(
                          child: new Column(
                            children: [
                              new Row(
                                children: <Widget>[
                                  new Radio<int>(
                                      value: ans1Value,
                                      groupValue: radioValue,
                                      onChanged: handleRadioValueChanged),
                                  new Text(
                                      "${questionBank[_counter.toString()]["ans1"]}")
                                ],
                              ),
                              new Divider(),
                              new Row(
                                children: <Widget>[
                                  new Radio<int>(
                                      value: ans2Value,
                                      groupValue: radioValue,
                                      onChanged: handleRadioValueChanged),
                                  new Text(
                                      "${questionBank[_counter.toString()]["ans2"]}")
                                ],
                              ),
                              new Divider(),
                              new Row(
                                children: <Widget>[
                                  new Radio<int>(
                                      value: ans3Value,
                                      groupValue: radioValue,
                                      onChanged: handleRadioValueChanged),
                                  new Text(
                                      "${questionBank[_counter.toString()]["ans3"]}")
                                ],
                              ),
                              new Divider(),
                              new Row(
                                children: <Widget>[
                                  new Radio<int>(
                                      value: ans4Value,
                                      groupValue: radioValue,
                                      onChanged: handleRadioValueChanged),
                                  new Text(
                                      "${questionBank[_counter.toString()]["ans4"]}")
                                ],
                              ),
                            ],
                          ),
                        ),
                      ),
                    ),
                  ],
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}