Setter of null value is not changing the value

[WissamALSbenaty]

In this case the setter works correctly if the parameter is null , so the initial value of the Observable class is null , but if it is initialised by a value , it will not change to null , in my case im deselect the option and the value still not null

Even if I change the value by passing null directly without a variable or parameter

part 'duty_officer_search_controller.g.dart';

@injectable
class DutyOfficerSearchController extends DutyOfficerSearchControllerBase with _$DutyOfficerSearchController {
  DutyOfficerSearchController(
    @factoryParam super.preselectedDutyOfficer,
   
  );
}

abstract class DutyOfficerSearchControllerBase extends BaseController with SearchingMixin, Store {
  final EmployeeModel? preselectedDutyOfficer;

  DutyOfficerSearchControllerBase(
    this.preselectedDutyOfficer,
  );

@observable
  late EmployeeModel? selectedDutyOfficer=preselectedDutyOfficer;

  @action
  void changeSelectedDutyOfficer(final EmployeeModel? value) {
    selectedDutyOfficer = value;
    if(value==null) {
      print('here null');
      selectedDutyOfficer=null;
    }
    print(' $selectedDutyOfficer $value');
  }


}