radio button diappear flutter code example

Example: flutter radio button

@overrideWidget build(BuildContext context) {  return new MaterialApp(      home: new Scaffold(          appBar: AppBar(            title: new Text('Kids Quiz App'),            centerTitle: true,            backgroundColor: Colors.blue,          ),          body: new Container(              padding: EdgeInsets.all(8.0),              child: new Column(                  mainAxisAlignment: MainAxisAlignment.center,                  children: [                    new Text(                      'Select correct answers from below:',                      style: new TextStyle(                          fontSize: 20.0, fontWeight: FontWeight.bold),                    ),                    new Padding(                      padding: new EdgeInsets.all(8.0),                    ),                    new Divider(height: 5.0, color: Colors.black),                    new Padding(                      padding: new EdgeInsets.all(8.0),                    ),                    new Text(                      'Lion is :',                      style: new TextStyle(                        fontWeight: FontWeight.bold,                        fontSize: 18.0,                      ),                    ),                    new Row(                      mainAxisAlignment: MainAxisAlignment.center,                      children: [                        new Radio(                          value: 0,                          groupValue: _radioValue1,                          onChanged: _handleRadioValueChange1,                        ),                        new Text(                          'Carnivore',                          style: new TextStyle(fontSize: 16.0),                        ),                        new Radio(                          value: 1,                          groupValue: _radioValue1,                          onChanged: _handleRadioValueChange1,                        ),                        new Text(                          'Herbivore',                          style: new TextStyle(                            fontSize: 16.0,                          ),                        ),                        new Radio(                          value: 2,                          groupValue: _radioValue1,                          onChanged: _handleRadioValueChange1,                        ),                        new Text(                          'Omnivore',                          style: new TextStyle(fontSize: 16.0),                        ),                      ],                    ),