I want the item I selected from dropDownMenu to be changed by default. How can I do it? The code I wrote is not working.

Code:

import 'package:flutter/material.dart';
import 'package:flutter_emoji/flutter_emoji.dart';

class arayuzEkrani extends StatefulWidget {

  @override
  State<arayuzEkrani> createState() => _arayuzEkraniState();
}

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