how to use 'dropdownbutton'?

There was an error when I inserted the DropdownButton code into my code. Outside the code containing the body, they declared it as a class, and when I put the class declared in the code, an error message appeared as below.

'_AssertionError ('package:flutter/src/material/dropdown.dart': Failed assertion: line 890 pos 15: 'items == null || items.isEmpty || value == null ||
              items.where((DropdownMenuItem<T> item) {
                return item.value == value;
              }).length == 1': There should be exactly one item with [DropdownButton]'s value: sex. 
Either zero or 2 or more [DropdownMenuItem]s were detected with the same value)'

Below is my code.

.....
....
onChanged: (_) {
                setState(() {});
              }
            ),

            SelectButton(),


          ],
          
         ),
        ),
  

class SelectButtonState extends State<SelectButton> {
  final List<String> _valueList = ['M', 'F'];
  String _selectedValue = 'sex';

  @override
  Widget build(BuildContext context) {
    return DropdownButton<String>(
      value: _selectedValue,
      items: _valueList.map((value) {
        return DropdownMenuItem(
          value: value,
          child: Text(value),
        );
      }).toList(),
      onChanged: (value) {
        setState(() {
          _selectedValue = value!;
        });
      },
    );
  }
}

class SelectButton extends StatefulWidget {
  const SelectButton ({Key? key}) : super(key: key);

  @override
  State<SelectButton> createState() => SelectButtonState();
}

I want to make sex select button...

Your _valueList contains ['M', 'F'] only and you are creating a DropDownButton out of it. When compiler finds initial value as "Select Sex" which is not available in the _valueList array, you get NULL error.

Solution -> Use 'Select Sex' as dropdown hint. Keep _selectedValue as null in intial declaration so that hint will be displayed.

Setting _selectedValue as null with null check:

String? _selectedValue = null;