In the dropDownButton in the appBar, I want the progress bar to appear when the item is changed. I wrote a code for this, but the progress Bar is not coming.

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

  @override
  _arayuzEkraniState createState() => _arayuzEkraniState();
}

class _arayuzEkraniState extends State<arayuzEkrani> {
  Map<String, String> countryFlags = {
    "usa": "