Flutter DropdownButtonFormField cause AssertionError

I want to create a DropdownButtonFormField and here is my code:

child: Row(
  children: [
    Flexible(
      flex: 1,
      child: DropdownButtonFormField(
          onChanged: (value) {},
          items: const [
            DropdownMenuItem(
              child: Text('men'),
            ),
            DropdownMenuItem(
              child: Text('women'),
            )
          ],
          decoration: const InputDecoration(
              labelText: 'gender',
              labelStyle: TextStyle(color: Color(0XFF121826)),
              enabledBorder: OutlineInputBorder(
                  borderSide:
                      BorderSide(color: Color(0XFF9B9B9B)),
                  borderRadius:
                      BorderRadius.all(Radius.circular(8))),
              focusedBorder: OutlineInputBorder(
                  borderSide:
                      BorderSide(color: Color(0XFF121826)),
                  borderRadius:
                      BorderRadius.all(Radius.circular(8))))),
    ),
    const SizedBox(width: 16),
    Flexible(
      flex: 1,
      child: TextFormField(
          controller: TextEditingController(),
          decoration: const InputDecoration(
              labelText: 'birth date',
              labelStyle: TextStyle(color: Color(0XFF121826)),
              enabledBorder: OutlineInputBorder(
                  borderSide:
                      BorderSide(color: Color(0XFF9B9B9B)),
                  borderRadius:
                      BorderRadius.all(Radius.circular(8))),
              focusedBorder: OutlineInputBorder(
                  borderSide:
                      BorderSide(color: Color(0XFF121826)),
                  borderRadius:
                      BorderRadius.all(Radius.circular(8)))),
          keyboardType: TextInputType.text),
    )
  ],
),

and as a result I get this:

Assertion failed: file:///home/.../dropdown.dart:1260:12
widget.items!.where((DropdownMenuItem<T> item) => item.value == widget.value).length == 1
is not true

how can I help this?

Try adding unique values to your DropdownMenuItems

items: const [
  DropdownMenuItem(
    child: Text('men'),
    value: 1,
  ),
  DropdownMenuItem(
    child: Text('women'),
    value: 2,
  )
],