There should be exactly one item with [DropdownButton]'s value: . Either zero or 2 or more

Question

I try to set value of dropdown button in Flutter. Actually, it takes item values from a List.

When I add value: _cStatus, in Dropdownbutton, I received the error message below:

There should be exactly one item with [DropdownButton]'s value: . Either zero or 2 or more [DropdownMenuItem]s were detected with the same value 'package:flutter/src/material/dropdown.dart': Failed assertion: line 1543 pos 15: 'items == null || items.isEmpty || value == null || items.where((DropdownMenuItem item) { return item.value == value; }).length == 1'

class _CheckinFormState extends State<CheckinForm> {

      final List<String> cStatusList = ['Checkin', 'CheckOut'];
    
      // form values
      String _cStatus='Checkin';

    
      @override
      Widget build(BuildContext context) {
        return Form(
          key: _formKey,
          child: Column(
            children: <Widget>[
              Text(
                'Update Your Checkin Status.',
                style: TextStyle(fontSize: 18.0),
              ),
              SizedBox(height: 20.0),
              DropdownButtonFormField(
                value: _cStatus, //my problem is here
                items: cStatusList.map((cstatus) {
                  return DropdownMenuItem(
                    value: cstatus,
                    child: Text('$cstatus'),
                  );
                }).toList(),
                onChanged: (val) {
                  setState(() {
                    _cStatus=val.toString();
                  });
                },
              ),
              SizedBox(height: 10.0),
              ElevatedButton(
                  child: Text(
                    'Update',
                    style: TextStyle(color: Colors.white),
                  ),
                  onPressed: () async {
                    ...

                  }),
            ],
          ),
        );
   }
}

Answer 1

After I added String type to DropdownButtonFormField as DropdownButtonFormField<String>, problem is solved.

          DropdownButtonFormField<String>(
            value: _cStatus,
            decoration: textInputDecoration,
            items: cStatusList.map((cstatus) {
              return DropdownMenuItem(
                value: cstatus,
                child: Text('$cstatus'),
              );
            }).toList(),
            onChanged: (val) {
              setState(() {
                _cStatus = val.toString();
              });
            },
          ),