在 null flutter 错误上调用了 getter 'Key'

Question

I am trying to achieve a screen with a Listview of items that can be selected with a Checkbox. This is the code for the main screen and checkbox:

class InterestHobbies extends StatefulWidget {
  @override
  _InterestHobbiesState createState() => _InterestHobbiesState();
}

class _InterestHobbiesState extends State<InterestHobbies> {

  final interests = [
    ModelCheckbox(title: 'Sports'),
    ModelCheckbox(title: 'Sports'),
    ModelCheckbox(title: 'Sports'),
    ModelCheckbox(title: 'Sports'),
    ModelCheckbox(title: 'Sports'),
    ModelCheckbox(title: 'Sports'),
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: ListView(
        children: [
          ...interests.map((interest) => null).toList(),
        ],
      ),
    );
  }

  Widget buildSingleCheckbox(ModelCheckbox interest) => buildCheckbox(
    interest: interest,
    onClicked: (){
      setState(() {
        final newValue = !interest.value;
        interest.value = newValue;
      });
    }
  );

  Widget buildCheckbox({
    @required ModelCheckbox interest,
    @required VoidCallback onClicked,
  }) =>
      ListTile(
        onTap: onClicked,

        leading: Checkbox(
          value: interest.value,
          onChanged: (value) => onClicked(),
        ),
        title: Text(interest.title),
      );
}

I also created a model file:

class ModelCheckbox {
  String title;
  bool value;

  ModelCheckbox({
    @required this.title,
    this.value = false,
  });
}

I get the following error:

The getter 'key' was called on null.
Receiver: null
Tried calling: key

Null check operator used on a null value

Does anyone know why? I am passing the value as interest and I created the model accordingly to be able to show the title dynamically and not hard coded.

Answer 1

You are mapping each element of the interests list to null. The ListView can only have Widgets as its children.

Try

  body: ListView(
    children: interests.map((interest) => buildSingleCheckbox(interest)).toList(),
  ),