add multiple dropdown value to a List in flutter - flutter

Question

In my flutter app I've created a dropdown. initially there is only one dropdown but the user can add more. the items of these dropdowns are the same. but the selected results has to be different. but my code is returning the last selected value. so how can I add it on list?
Model
class DropModel {
String? selected;
DropModel({this.selected});
setData(list) {
for (int i = 0; i < list.length; i++) {
selected = list;
}
}
}
initialize
List<String>? selCat;
DropModel selDrop = DropModel();
dropdown widget
DropdownButtonFormField2 _generatedDropDown(
List<String> category, String? selected, int index) {
final group = _GroupControllers();
return DropdownButtonFormField2(
decoration: InputDecoration(
isDense: true,
contentPadding: EdgeInsets.zero,
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(5),
),
),
isExpanded: true,
hint: const Text(
'Select Category',
style: TextStyle(fontSize: 14),
),
icon: const Icon(
Icons.arrow_drop_down,
color: Colors.black45,
),
iconSize: 30,
buttonHeight: 60,
buttonPadding: const EdgeInsets.only(left: 20, right: 10),
items: category
.map((item) => DropdownMenuItem<String>(
value: item,
child: Text(
item,
style: const TextStyle(
fontSize: 14,
),
),
))
.toList(),
validator: (value) {
if (value == null) {
return 'Please select Catagory.';
}
},
onChanged: (value) {
//Do something when changing the item if you want.
setState(() {
// selDrop.setData(value.toString());
selected = value.toString();
selDrop.selected = selected;
selCat!.insert(index, selected.toString());
});
},
onSaved: (value) {
//selDrop.setData(value.toString());
selected = value.toString();
selDrop.selected = selected;
selCat!.insert(index, selected.toString());
},
);
}
result looped
print(".................${selDrop.selected}"); // returns the last added/ selected value
print(".................${selCat}"); //returns null
the traditional List returns null and the model returns the last added/selected value. how can i add multiple dropdown selected values to a List? each has different values. in a textfield I could create dynamic TextEditingController but how can I manage dropdowns