Flutter save the selected item from a dropdownmenu into a variable - flutter

Question

i want to save my selected item form a dropdown menu into a variable, that i can use my selection in an other file to show it there in a dialog box.
Is there a simple way how i can save the selection in a var or someone knew a better way to get my selection into the other file?
import 'package:flutter/material.dart';
import 'package:dropdown_search/dropdown_search.dart';
var AderOutput;
class Ader extends StatefulWidget {
#override
_AderState createState() => _AderState();
}
class _AderState extends State<Ader> {
#override
Widget build(BuildContext context) {
return Container(
padding: EdgeInsets.all(25),
child: DropdownSearch<String>(
validator: (v) => v == null ? "Erforderliches Feld" : null,
hint: "Wähle eine Ader",
mode: Mode.MENU,
showSelectedItem: true,
items: ["Ader 1", "Ader 2"],
label: "Wähle eine Ader",
showClearButton: true,
onChanged: print,
popupItemDisabled: (String s) => s.startsWith('I'),
selectedItem: null,
onBeforeChange: (a, b) {
if (b == null) {
AlertDialog alert = AlertDialog(
title: Text('Sind Sie sicher,'),
content: Text('dass Sie die Auswahl zurücksetzten möchten?'),
actions: [
TextButton(
child: Text("Ja"),
onPressed: () {
Navigator.of(context).pop(true);
},
),
TextButton(
child: Text("Nein"),
onPressed: () {
Navigator.of(context).pop(false);
},
),
],
);
return showDialog<bool>(
context: context,
builder: (BuildContext context) {
return alert;
});
}
return Future.value(true);
},
),
);
}
}