Fix: Null Reference Exception in ShellContentFragment.Destroy (#29713)

* Fix null reference exception when removing appearance observer in ShellContentFragment

* Change to use `as` syntax

* Add test to verify ShellContentFragment.Destroy handles null _shellContext gracefully

* Add test to ensure ShellContentFragment.Destroy handles null _shellContext gracefully

* Add safeguard in ShellContentFragment.Destroy to prevent multiple calls

Author: postalservice14

src/Controls/src/Core/Compatibility/Handlers/Shell/Android/ShellContentFragment.cs

@@ -71,6 +71,7 @@ void IAppearanceObserver.OnAppearanceChanged(ShellAppearance appearance)
 		AToolbar _toolbar;
 		IShellToolbarTracker _toolbarTracker;
 		bool _disposed;
+		bool _destroyed;
 
 		public ShellContentFragment(IShellContext shellContext, ShellContent shellContent)
 		{
@@ -167,13 +168,18 @@ public override AView OnCreateView(LayoutInflater inflater, ViewGroup container,
 
 		void Destroy()
 		{
+			if (_destroyed)
+				return;
+			
+			_destroyed = true;
+			
 			// If the user taps very quickly on back button multiple times to pop a page,
 			// the app enters background state in the middle of the animation causing the fragment to be destroyed without completing the animation.
 			// That'll cause `IAnimationListener.onAnimationEnd` to not be called, so we need to call it manually if something is still subscribed to the event
 			// to avoid the navigation `TaskCompletionSource` to be stuck forever.
 			AnimationFinished?.Invoke(this, EventArgs.Empty);
 
-			((IShellController)_shellContext.Shell).RemoveAppearanceObserver(this);
+			(_shellContext?.Shell as IShellController)?.RemoveAppearanceObserver(this);
 
 			if (_shellContent != null)
 			{

src/Controls/tests/DeviceTests/Elements/Shell/ShellTests.Android.cs

@@ -516,6 +516,61 @@ await CreateHandlerAndAddToWindow<ShellRenderer>(shell, (handler) =>
 			});
 		}
 
+		[Fact(DisplayName = "ShellContentFragment.Destroy handles null _shellContext gracefully")]
+		public async Task ShellContentFragmentDestroyHandlesNullShellContext()
+		{
+			SetupBuilder();
+
+			var shell = await CreateShellAsync(shell =>
+			{
+				shell.Items.Add(new TabBar()
+				{
+					Items =
+					{
+						new ShellContent()
+						{
+							Route = "Item1",
+							Content = new ContentPage { Title = "Page 1" }
+						},
+						new ShellContent()
+						{
+							Route = "Item2", 
+							Content = new ContentPage { Title = "Page 2" }
+						},
+					}
+				});
+			});
+
+			await CreateHandlerAndAddToWindow<ShellHandler>(shell, async (handler) =>
+			{
+				await OnLoadedAsync(shell.CurrentPage);
+				await OnNavigatedToAsync(shell.CurrentPage);
+
+				// Navigate to trigger fragment creation
+				await shell.GoToAsync("//Item2");
+				await OnNavigatedToAsync(shell.CurrentPage);
+
+				// Test normal destruction - should work without issues
+				await shell.GoToAsync("//Item1");
+				await OnNavigatedToAsync(shell.CurrentPage);
+
+				// Test null context scenario
+				var exception = Record.Exception(() => 
+				{
+					// Create fragment with null context - this should not throw
+					Page page = new ContentPage();
+					var fragment = new ShellContentFragment((IShellContext)null, page);
+			
+					// Dispose the fragment which calls Destroy internally
+					// This validates the null-conditional operators in Destroy method
+					fragment.Dispose();
+				});
+
+				// Verify no exception was thrown - validates (_shellContext?.Shell as IShellController)?.RemoveAppearanceObserver(this);
+				Assert.Null(exception);
+			});
+		}
+
 		protected AView GetFlyoutPlatformView(ShellRenderer shellRenderer)
 		{
 			var drawerLayout = GetDrawerLayout(shellRenderer);