How to solve the error: Null check operator used on a null value?

Question

字段验证不起作用，显示错误：对空值使用了空检查运算符。我从 docs.flutter 中举了一个例子。我该如何解决？

class _AddGroupPageState extends State<AddGroupPage> {
  final _groupController = TextEditingController();
  final _formKey = GlobalKey<FormState>();
  @override
  void dispose() {
    _groupController.dispose();
    super.dispose();
  }
  @override
  Widget build(BuildContext context) {
    return: Scaffold(
    body: Padding(
      padding: const EdgeInsets.all(8.0),
      child: Column(
        children: [
          TextFormField(
            key: _formKey,
            controller: _groupController,
            validator: (value) {
              if (value == null || value.isEmpty) {
                return 'Group name cannot be empty';
              }
              return null;
            },
          ),
        ],
      ),
    ),
    ...

void _saveGroup() {
  final isValid = _formKey.currentState!.validate();
  if (isValid) {
    BlocProvider.of<NewGroupBloc>(context)
        .add(AddGroupEvent(_groupController.text.trim()));
  }
}

Answer 1

用 Form 小部件包装您的 TextFormField 并将 formkey 传递给该小部件

 Form(
          key: _formKey,
          child: TextFormField(
            controller: _groupController,
            validator: (value) {
              if (value == null || value.isEmpty) {
                return 'Group name cannot be empty';
              }
              return null;
            },
          ),
        ),

Answer 2

您必须将 formKey 提供给表单而不是文本字段本身像这样使用：

                    Form(
                    key: _formKey,
                    child: Column(
                      children: [
                        TextFormField(
                        
                        //your code

How to solve the error: Null check operator used on a null value?

How to solve the error: Null check operator used on a null value?

dart

flutter