fix: Ensure app is started after native WPF template is applied

If the initial page of the application has an input field as the first focusable element, an exception would occur, as the native overlay layer was not initialized yet. Moving app startup after WPF's OnApplyTemplate ensures that this condition does not occur.

Author: MartinZikmund

src/Uno.UI.Runtime.Skia.Wpf/WpfHost.cs

@@ -46,7 +46,10 @@ public class WpfHost : WpfControl, WinUI.ISkiaHost
 		private const string NativeOverlayLayerPart = "NativeOverlayLayer";
 
 		private readonly bool designMode;
-
+		private readonly Func<UnoApplication> _appBuilder;
+		
+		private bool _appStarted = false;
+		
 		[ThreadStatic] private static WpfHost _current;
 
 		private WpfCanvas? _nativeOverlayLayer = null;
@@ -99,20 +102,13 @@ public WpfHost(global::System.Windows.Threading.Dispatcher dispatcher, Func<WinU
 		public WpfHost(global::System.Windows.Threading.Dispatcher dispatcher, Func<WinUI.Application> appBuilder)
 		{
 			_current = this;
+			_appBuilder = appBuilder;
 
 			designMode = DesignerProperties.GetIsInDesignMode(this);
 
-			void CreateApp(WinUI.ApplicationInitializationCallbackParams _)
-			{
-				var app = appBuilder();
-				app.Host = this;
-			}
-
 			Windows.UI.Core.CoreDispatcher.DispatchOverride = d => dispatcher.BeginInvoke(d);
 			Windows.UI.Core.CoreDispatcher.HasThreadAccessOverride = dispatcher.CheckAccess;
 
-			WinUI.Application.StartWithArguments(CreateApp);
-
 			WinUI.Window.InvalidateRender += () =>
 			{
 				InvalidateOverlays();
@@ -156,6 +152,26 @@ public override void OnApplyTemplate()
 			base.OnApplyTemplate();
 
 			_nativeOverlayLayer = GetTemplateChild(NativeOverlayLayerPart) as WpfCanvas;
+
+			// App needs to be created after the native overlay layer is properly initialized
+			// otherwise the initially focused input element would cause exception.
+			StartApp();
+		}
+
+		private void StartApp()
+		{
+			if (_appStarted)
+			{
+				return;
+			}
+			
+			void CreateApp(WinUI.ApplicationInitializationCallbackParams _)
+			{
+				var app = _appBuilder();
+				app.Host = this;
+			}
+
+			WinUI.Application.StartWithArguments(CreateApp);
 		}
 
 		private void MainWindow_StateChanged(object? sender, EventArgs e)