[BUG] Crash on second window close when SnackBar enabled on Windows (still reproducible in v12.0.0)

[slovan]

Is there an existing issue for this?

• I have searched the existing issues

Did you read the "Reporting a bug" section on Contributing file?

• I have read the "Reporting a bug" section on Contributing file: https://github.com/CommunityToolkit/Maui/blob/main/CONTRIBUTING.md#reporting-a-bug

Current Behavior

A .NET MAUI app using multiple windows and enabling snackbar notifications for Windows via:
builder.UseMauiApp<App>().UseMauiCommunityToolkit(options => options.SetShouldEnableSnackbarOnWindows(true));

On the second window close (i.e., after opening, closing, reopening, and then closing the secondary window), the app crashes with an unhandled exception:

Element not found.
Not Registered for App Notifications!

and this stack trace:

at WinRT.ExceptionHelpers.g__Throw|38_0(Int32 hr)
at ABI.Microsoft.Windows.AppNotifications.IAppNotificationManagerMethods.Unregister(IObjectReference _obj)
at Microsoft.Windows.AppNotifications.AppNotificationManager.Unregister()
at CommunityToolkit.Maui.Options.<>c.b__22_4(Window _, WindowEventArgs ) in //src/CommunityToolkit.Maui/Options.cs:line 93
at Microsoft.Maui.MauiWinUIWindow.<>c__DisplayClass10_0.b__0(OnClosed del)
at Microsoft.Maui.LifecycleEvents.LifecycleEventServiceExtensions.InvokeLifecycleEvents[TDelegate](IServiceProvider services, Action`1 action)
at Microsoft.Maui.MauiWinUIWindow.OnClosed(Object sender, WindowEventArgs args)
at Microsoft.Maui.MauiWinUIWindow.OnClosedPrivate(Object sender, WindowEventArgs args)
at WinRT.EventSource_global__Windows_Foundation_TypedEventHandler_object__global__Microsoft_UI_Xaml_WindowEventArgs.EventState.b__1_0(Object sender, WindowEventArgs args)
at WinRT.GenericTypeInstantiations.Windows_Foundation_TypedEventHandler_2_object__Microsoft_UI_Xaml_WindowEventArgs.Do_Abi_Invoke(IntPtr thisPtr, IntPtr sender, IntPtr args)

Expected Behavior

The second window should open and close repeatedly without throwing an exception.

Steps To Reproduce

1. Create a MAUI app and enable Windows Snackbar in MauiProgram.cs (or use the sample project linked to this ticket).
2. Launch the app.
3. Open the secondary window (via the “Click me” button in the sample).
4. Close that window.
5. Reopen the secondary window.
6. Close it again - and observe the crash on the second close.

Link to public reproduction project repository

https://github.com/slovan/MultiWindowIssueRepro

Environment

- .NET MAUI CommunityToolkit: 12.0.0
- OS: Windows 11 Build 10.0.26100
- .NET MAUI: 9.0.301

Anything else?

• The exception occurs only on the second window close, not the first.
• The original issue Application crashes upon closing a second window when using Builder.options.SetShouldEnableSnackbarOnWindows(true)  #2279 was closed automatically when PR Fix Windows Media Element in MultiWindow Mode on exit Crash #2259 was merged. However, the crash still occurs in 12.0.0.
• I have reproduced the issue with the following CommunityToolkit.Maui versions in addition to the mentioned above: 11.0.0, 11.1.0, 11.1.1 and 11.2.0.
• The last known working version where this did not occur is 10.0.0.

[ne0rrmatrix]

I can verify this bug is still present. I am working on a PR to fix this. It should be ready in a few hours.