Fix: Fixed issue where the app sometimes got stuck on splash screen (#13013)

Author: hishitetsu

src/Files.App/MainWindow.xaml.cs

@@ -213,7 +213,7 @@ async Task PerformNavigation(string payload, string selectItem = null)
 					LeftPaneSelectItemParam = selectItem,
 				};
 
-				if (rootFrame.Content is not null)
+				if (rootFrame.Content is MainPage)
 					await mainPageViewModel.AddNewTabByParam(typeof(PaneHolderPage), paneNavigationArgs);
 				else
 					rootFrame.Navigate(typeof(MainPage), paneNavigationArgs, new SuppressNavigationTransitionInfo());