Fix unattached MenuController throws assertion error when calling MenuController.close() (#156588)
Fixes [Closing a menu controller that was never attached throws an assertion error](https://github.com/flutter/flutter/issues/156572)
### Code sample
<details>
<summary>expand to view the code sample</summary>
```dart
import 'package:flutter/material.dart';
void main() => runApp(const MyApp());
class MyApp extends StatelessWidget {
const MyApp({super.key});
@override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
useMaterial3: true,
),
home: const MyHomePage(),
);
}
}
class MyHomePage extends StatefulWidget {
const MyHomePage({super.key});
@override
State<MyHomePage> createState() => _MyHomePageState();
}
class _MyHomePageState extends State<MyHomePage> {
@override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: ElevatedButton(
onPressed: () => Navigator.of(context).push(
MaterialPageRoute(
builder: (context) => const MenuControllerPage(),
),
),
child: const Text('Press Me'),
),
),
);
}
}
class MenuControllerPage extends StatefulWidget {
const MenuControllerPage({super.key});
@override
State<MenuControllerPage> createState() => _MenuControllerPageState();
}
class _MenuControllerPageState extends State<MenuControllerPage> {
// This menu controller is intentionally left as not attached to a MenuAnchor.
final MenuController _menuController = MenuController();
@override
void dispose() {
// Close the menu when the page is popped.
_menuController.close();
super.dispose();
}
@override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: ElevatedButton(
child: const Text('Go back'),
onPressed: () => Navigator.of(context).pop(),
),
),
);
}
}
```
</details>
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