Fixed a NullReferenceException when starting application with empty shell on Windows

src/Controls/src/Core/Compatibility/Handlers/Shell/iOS/ShellRenderer.cs

@@ -364,7 +364,7 @@ void SetupCurrentShellItem()
 		{
 			if (Shell.CurrentItem == null)
 			{
-				return;
+				throw new InvalidOperationException($"Content not found for active {Shell}. Title: {Shell.Title}. Route: {Shell.Route}.");
 			}
 			else if (_currentShellItemRenderer == null)
 			{

src/Controls/src/Core/Handlers/Shell/Windows/ShellView.cs

@@ -207,6 +207,12 @@ public override bool Equals(object obj) =>
 
 		internal void SwitchShellItem(ShellItem newItem, bool animate = true)
 		{
+			if (newItem is null)
+				throw new InvalidOperationException("Active Shell Item not set. Have you added any Shell Items to your Shell?");
+
+			if (newItem.CurrentItem is null)
+				throw new InvalidOperationException($"Content not found for active {newItem}. Title: {newItem.Title}. Route: {newItem.Route}.");
+
 			var navItems = FlyoutItems.OfType<NavigationViewItemViewModel>();
 
 			// Implicit items aren't items that are surfaced to the user 

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

@@ -1139,6 +1139,23 @@ await CreateHandlerAndAddToWindow<IWindowHandler>(shell, _ =>
 			});
 		}
 
+#if !ANDROID // On Android, the exception causes the app to terminate, resulting in a device test failure.
+		[Fact(DisplayName = "Initialize Empty Shell Throws InvalidOperationException")]
+		public async Task InitializeEmptyShellThrowsInvalidOperationException()
+		{
+			SetupBuilder();
+			var shell = new Shell();
+
+			await Assert.ThrowsAsync<InvalidOperationException>(async () =>
+			{
+				await CreateHandlerAndAddToWindow<ShellHandler>(shell, (handler) =>
+				{
+					return Task.CompletedTask;
+				});
+			});
+		}
+#endif
+
 		protected Task<Shell> CreateShellAsync(Action<Shell> action) =>
 			InvokeOnMainThreadAsync(() =>
 			{