i want to validation for pop menu item in flutter - database

Question

This is my code:
final List items = [ "Residential", "Commercial", ];
String? selectedValue;
i am using popmenuitemlist but i do not know how to validation used for popmenuitem
This is my Document Edit Page code
SizedBox(
height: MediaQuery.of(context).size.height *
0.05,
width:
MediaQuery.of(context).size.width * 0.4,
child: TextFormField(
autovalidateMode:
AutovalidateMode.onUserInteraction,
controller: docTypeController =
TextEditingController(
text: '${snapshot.data!.docType}',
),
decoration: InputDecoration(
suffixIcon: PopupMenuButton<String>(
icon: Icon(Icons.arrow_drop_down),
onSelected: (String value) {
docTypeController.text = value;
},
itemBuilder: (BuildContext context) {
return items
.map<PopupMenuItem<String>>(
(String value) {
return new PopupMenuItem(
padding: EdgeInsets.symmetric(
horizontal: 45,
vertical: 5,
),
textStyle: TextStyle(
color: Colors.black,
fontSize: 10),
child: new Text(value),
value: value);
}).toList();
},
),
border: OutlineInputBorder(),
labelText: 'Document Type',
),
validator: MultiValidator([
RequiredValidator(
errorText: 'Required*')
]),
),
),
],
),
),