Fix Windows Media Element in MultiWindow Mode on exit Crash (CommunityToolkit#2259)

* Track window count and manage snackbar notifications

Added a static `NumberOfWindows` property to the `Options` class to track the number of windows. Updated `SetShouldEnableSnackbarOnWindows` to increment/decrement `NumberOfWindows` on window creation/closure. Conditional registration/unregistration of `AppNotificationManager` for snackbar notifications based on `NumberOfWindows` count.

* Remove `NumberOfWindows `

* Add null check

---------

Co-authored-by: Brandon Minnick <13558917+brminnick@users.noreply.github.com>

Author: ne0rrmatrix

src/CommunityToolkit.Maui/Options.cs

@@ -70,19 +70,27 @@ public void SetShouldEnableSnackbarOnWindows(bool value)
 				events.AddWindows(windows => windows
 					.OnLaunched((_, _) =>
 					{
-						Microsoft.Windows.AppNotifications.AppNotificationManager.Default.NotificationInvoked += OnSnackbarNotificationInvoked;
-						Microsoft.Windows.AppNotifications.AppNotificationManager.Default.Register();
+						if (Application.Current is null)
+						{
+							throw new InvalidOperationException($"{nameof(Application)}.{nameof(Application.Current)} cannot be null when Windows are launched");
+						}
+
+						else if (Application.Current.Windows.Count is 1)
+						{
+							Microsoft.Windows.AppNotifications.AppNotificationManager.Default.NotificationInvoked += OnSnackbarNotificationInvoked;
+							Microsoft.Windows.AppNotifications.AppNotificationManager.Default.Register();
+						}
 					})
 					.OnClosed((_, _) =>
 					{
-						try
+						if (Application.Current is null)
 						{
-							Microsoft.Windows.AppNotifications.AppNotificationManager.Default.NotificationInvoked -= OnSnackbarNotificationInvoked;
-							Microsoft.Windows.AppNotifications.AppNotificationManager.Default.Unregister();
+							throw new InvalidOperationException($"{nameof(Application)}.{nameof(Application.Current)} cannot be null when Windows are closed");
 						}
-						catch
+						else if (Application.Current.Windows.Count is 1)
 						{
-							// And Element not found exception may be thrown when unregistering the event handler after using MediaElement accross multiple Windows
+							Microsoft.Windows.AppNotifications.AppNotificationManager.Default.NotificationInvoked -= OnSnackbarNotificationInvoked;
+							Microsoft.Windows.AppNotifications.AppNotificationManager.Default.Unregister();
 						}
 					}));
 			});