Fix: Fixed issue where the app would crash when opening the propertie…

…s window twice (files-community#14386)

src/Files.App/Helpers/UI/DragZoneHelper.cs

@@ -25,7 +25,12 @@ public static void RaiseSetTitleBarDragRegion(this Window window, SetTitleBarDra
 			// UIElement.RasterizationScale is always 1
 			var source = InputNonClientPointerSource.GetForWindowId(window.AppWindow.Id);
 			var uiElement = window.Content;
-			var scaleFactor = uiElement.XamlRoot.RasterizationScale;
+			var xamlRoot = uiElement?.XamlRoot;
+
+			if (xamlRoot is null)
+				return;
+
+			var scaleFactor = xamlRoot.RasterizationScale;
 			var size = window.AppWindow.Size;
 			// If the number of regions is 0 or 1, AppWindow will automatically reset to the default region next time, but if it is >=2, it will not and need to be manually cleared
 			source.ClearRegionRects(NonClientRegionKind.Passthrough);

src/Files.App/Views/Properties/MainPropertiesPage.xaml.cs

@@ -52,7 +52,7 @@ private void Page_Loaded(object sender, RoutedEventArgs e)
 
 			UpdatePageLayout();
 			Window.RaiseSetTitleBarDragRegion(SetTitleBarDragRegion);
-			Window.AppWindow.Changed += (_, _) => Window.RaiseSetTitleBarDragRegion(SetTitleBarDragRegion);
+			Window.AppWindow.Changed += AppWindow_Changed;
 		}
 
 		private int SetTitleBarDragRegion(InputNonClientPointerSource source, SizeInt32 size, double scaleFactor, Func<UIElement, RectInt32?, RectInt32> getScaledRect)
@@ -116,6 +116,7 @@ private void Window_Closed(object sender, WindowEventArgs args)
 		{
 			AppSettings.ThemeModeChanged -= AppSettings_ThemeModeChanged;
 			Window.Closed -= Window_Closed;
+			Window.AppWindow.Changed -= AppWindow_Changed;
 
 			if (MainPropertiesViewModel.ChangedPropertiesCancellationTokenSource is not null &&
 				!MainPropertiesViewModel.ChangedPropertiesCancellationTokenSource.IsCancellationRequested)
@@ -124,6 +125,11 @@ private void Window_Closed(object sender, WindowEventArgs args)
 			}
 		}
 
+		private void AppWindow_Changed(AppWindow sender, AppWindowChangedEventArgs e)
+		{
+			Window.RaiseSetTitleBarDragRegion(SetTitleBarDragRegion);
+		}
+
 		public override async Task<bool> SaveChangesAsync()
 			=> await Task.FromResult(false);