Code Quality: Added null check to UIHelpers.CloseAllDialogs (#16059)

Author: yaira2

src/Files.App (Package)/Package.appxmanifest

@@ -16,7 +16,7 @@
     <Identity
         Name="FilesDev"
         Publisher="CN=Files"
-        Version="3.6.2.0" />
+        Version="3.6.3.0" />
 
     <Properties>
         <DisplayName>Files - Dev</DisplayName>

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

@@ -92,15 +92,14 @@ private static ContentDialog SetContentDialogRoot(ContentDialog contentDialog)
 
 		public static void CloseAllDialogs()
 		{
+			if (MainWindow.Instance?.Content?.XamlRoot == null)
+				return;
+
 			var openedDialogs = VisualTreeHelper.GetOpenPopupsForXamlRoot(MainWindow.Instance.Content.XamlRoot);
 
 			foreach (var item in openedDialogs)
-			{
 				if (item.Child is ContentDialog dialog)
-				{
 					dialog.Hide();
-				}
-			}
 		}
 
 		private static IEnumerable<IconFileInfo> SidebarIconResources = LoadSidebarIconResources();