Flutter DropdownButton returns error for onChange event

Question

I'm writing a project and wants to add a DropDownButton but it always return error once I add onCnange event. Dropdown always returns error. I don't know what's wrong with it.

@override
  Widget build(BuildContext context) {
    return Material( child: Column( children: [
      DropdownButton(items: [], value: dropdownValue, onChanged: (String value)  {
        setState(() {
          dropdownValue = value;
        });
      },)
    ], ), );
  }

Error message

The argument type 'void Function(String)' can't be assigned to the parameter type 'void Function(Object?)?'.

Solution

function and value might be nullable, so either remove String type from the callback or use the nullable type.

Solution 1

DropdownButton<String>(
  items: [],
  value: dropdownValue,
  onChanged: (value) {
    setState(() {
      dropdownValue = value;
    });
  },
)

Solution 2

DropdownButton<String>(
  items: [],
  value: dropdownValue,
  onChanged: (String? value) {
    setState(() {
      dropdownValue = value;
    });
  },
)