Make sure to account for a null AppWindow

eng/BannedSymbols.txt

@@ -1,4 +1,5 @@
 M:Microsoft.Extensions.DependencyInjection.Extensions.ServiceCollectionDescriptorExtensions.TryAddSingleton`2(Microsoft.Extensions.DependencyInjection.IServiceCollection);Use a Factory method to create the service instead
 M:Android.Content.Res.ColorStateList.#ctor(System.Int32[][],System.Int32[]);Use Microsoft.Maui.PlatformInterop.Get*ColorStateList() Java methods instead
 P:Microsoft.Maui.MauiWinUIApplication.Services;Use the IPlatformApplication.Current.Services instead
-P:Microsoft.Maui.MauiWinUIApplication.Application;Use the IPlatformApplication.Current.Application instead
+P:Microsoft.Maui.MauiWinUIApplication.Application;Use the IPlatformApplication.Current.Application instead
+P:Microsoft.UI.Xaml.Window.AppWindow;This API doesn't have null safety. Use GetAppWindow() and make sure to account for the possibility that GetAppWindow() might be null.

src/Controls/samples/Controls.Sample.UITests/Issues/Issue17490.cs

@@ -0,0 +1,73 @@
+using System;
+using System.Threading.Tasks;
+using Microsoft.Maui;
+using Microsoft.Maui.Controls;
+using Microsoft.Maui.Platform;
+
+namespace Maui.Controls.Sample.Issues
+{
+	[Issue(IssueTracker.Github, 17490, "Crash using Pinvoke.SetParent to create Window as Child", PlatformAffected.UWP)]
+	public class Issue17490 : TestContentPage
+	{
+		Label successLabel;
+		protected override void Init()
+		{
+			successLabel = new Label() { Text = "Success", AutomationId = "SuccessLabel" };
+
+			Content = new VerticalStackLayout()
+			{
+				new Label()
+				{
+					Text = "This test validates that opening a new WinUI Window parented to this window won't crash."
+				}
+			};
+		}
+
+		protected override void OnNavigatedTo(NavigatedToEventArgs args)
+		{
+			base.OnNavigatedTo(args);
+
+			try
+			{
+				var myWindow = new MyWindow(new ContentPage());
+				myWindow.Page.Loaded += async (_, _) =>
+				{
+					await Task.Yield();
+					Application.Current.CloseWindow(myWindow);
+					await Task.Yield();
+					(this.Content as VerticalStackLayout)
+						.Add(successLabel);
+				};
+
+				Application.Current.OpenWindow(myWindow);
+			}
+			catch (Exception exc)
+			{
+				successLabel.Text = $"{exc}";
+			}
+		}
+
+		public class MyWindow : Window
+		{
+			public MyWindow(Page page) : base(page)
+			{
+			}
+
+#if WINDOWS
+			protected override void OnHandlerChanged()
+			{
+				base.OnHandlerChanged();
+				if (Handler is null)
+				{
+					return;
+				}
+
+				var mainWindowHandle = (Application.Current.MainPage.Window.Handler.PlatformView as MauiWinUIWindow).GetWindowHandle();
+				var childWindowHandle = (Handler.PlatformView as MauiWinUIWindow).GetWindowHandle();
+
+				Platform.PlatformMethods.SetParent(childWindowHandle, mainWindowHandle);
+			}
+#endif
+		}
+	}
+}

src/Controls/samples/Controls.Sample.UITests/Platforms/Windows/PlatformMethods.cs

@@ -0,0 +1,12 @@
+#nullable enable
+using System;
+using System.Runtime.InteropServices;
+
+namespace Maui.Controls.Sample.Platform
+{
+	static class PlatformMethods
+	{
+		[DllImport("user32.dll")]
+		public static extern IntPtr SetParent(IntPtr hWndChild, IntPtr hWndNewParent);
+	}
+}

src/Controls/tests/UITests/Tests/Issues/Issue17490.cs

@@ -0,0 +1,28 @@
+using System.Drawing;
+using Microsoft.Maui.Appium;
+using NUnit.Framework;
+using OpenQA.Selenium.Appium.MultiTouch;
+using TestUtils.Appium.UITests;
+
+namespace Microsoft.Maui.AppiumTests.Issues
+{
+	public class Issue17490 : _IssuesUITest
+	{
+		public Issue17490(TestDevice device) : base(device)
+		{
+		}
+
+		public override string Issue => "Crash using Pinvoke.SetParent to create Window as Child";
+
+		[Test]
+		public void AppDoesntCrashWhenOpeningWinUIWindowParentedToCurrentWindow()
+		{
+			UITestContext.IgnoreIfPlatforms(new[]
+			{
+				TestDevice.Mac, TestDevice.iOS, TestDevice.Android
+			});
+
+			App.WaitForElement("Success");
+		}
+	}
+}

src/Core/src/Handlers/Window/WindowHandler.Windows.cs

@@ -160,7 +160,10 @@ internal static void MapTitleBarDragRectangles(IWindowHandler handler, IWindow w
 			if (!AppWindowTitleBar.IsCustomizationSupported())
 				return;
 
-			var titleBar = handler.PlatformView.AppWindow.TitleBar;
+			var titleBar = handler.PlatformView.GetAppWindow()?.TitleBar;
+			if (titleBar is null)
+				return;
+
 			var titleBarRects = window.TitleBarDragRectangles;
 
 			if (titleBarRects is null)

src/Core/src/Platform/Windows/MauiWinUIWindow.cs

@@ -38,7 +38,13 @@ public MauiWinUIWindow()
 			// and then we can react accordingly
 			if (AppWindowTitleBar.IsCustomizationSupported())
 			{
-				base.AppWindow.TitleBar.ExtendsContentIntoTitleBar = true;
+				var titleBar = this.GetAppWindow()?.TitleBar;
+
+				if (titleBar is not null)
+				{
+					titleBar.ExtendsContentIntoTitleBar = true;
+				}
+
 				_viewSettings.ColorValuesChanged += _viewSettings_ColorValuesChanged;
 				SetTileBarButtonColors();
 			}
@@ -202,9 +208,14 @@ private void SetTileBarButtonColors()
 		{
 			if (AppWindowTitleBar.IsCustomizationSupported())
 			{
-				base.AppWindow.TitleBar.ButtonBackgroundColor = Colors.Transparent;
-				base.AppWindow.TitleBar.ButtonInactiveBackgroundColor = Colors.Transparent;
-				base.AppWindow.TitleBar.ButtonForegroundColor = _viewSettings.GetColorValue(ViewManagement.UIColorType.Foreground);
+				var titleBar = this.GetAppWindow()?.TitleBar;
+
+				if (titleBar is null)
+					return;
+
+				titleBar.ButtonBackgroundColor = Colors.Transparent;
+				titleBar.ButtonInactiveBackgroundColor = Colors.Transparent;
+				titleBar.ButtonForegroundColor = _viewSettings.GetColorValue(ViewManagement.UIColorType.Foreground);
 			}
 		}
 

src/Core/src/Platform/Windows/NavigationRootManager.cs

@@ -19,7 +19,11 @@ public NavigationRootManager(Window platformWindow)
 			_rootView.BackRequested += OnBackRequested;
 			_rootView.OnApplyTemplateFinished += WindowRootViewOnApplyTemplateFinished;
 
-			SetTitleBarVisibility(_platformWindow.AppWindow.TitleBar.ExtendsContentIntoTitleBar);
+			var titleBar = _platformWindow.GetAppWindow()?.TitleBar;
+			if (titleBar is not null)
+			{
+				SetTitleBarVisibility(titleBar.ExtendsContentIntoTitleBar);
+			}
 		}
 
 		internal void SetTitleBarVisibility(bool isVisible)
@@ -31,8 +35,12 @@ internal void SetTitleBarVisibility(bool isVisible)
 			var appbarHeight = isVisible ? 32 : 0;
 			if (isVisible && UI.Windowing.AppWindowTitleBar.IsCustomizationSupported())
 			{
-				var density = _platformWindow.GetDisplayDensity();
-				appbarHeight = (int)(_platformWindow.AppWindow.TitleBar.Height / density);
+				var titleBar = _platformWindow.GetAppWindow()?.TitleBar;
+				if (titleBar is not null)
+				{
+					var density = _platformWindow.GetDisplayDensity();
+					appbarHeight = (int)(titleBar.Height / density);
+				}
 			}
 
 			_rootView.UpdateAppTitleBar(

src/Core/src/Platform/Windows/WindowExtensions.cs

@@ -235,8 +235,6 @@ public static float GetDisplayDensity(this UI.Xaml.Window platformWindow)
 				return 1.0f;
 			}
 
-			var id = platformWindow.AppWindow.Id;
-
 			return PlatformMethods.GetDpiForWindow(hwnd) / DeviceDisplay.BaseLogicalDpi;
 		}