Fix: Fixed ArgumentNullException in AppUnhandledException (#13709)

Author: yaira2

src/Files.App/App.xaml.cs

@@ -472,7 +472,7 @@ private static void AppUnhandledException(Exception? ex, bool shouldShowNotifica
 			Debugger.Break();
 
 			SaveSessionTabs();
-			App.Logger.LogError(ex, ex.Message);
+			Logger.LogError(ex, ex?.Message ?? "An error occured");
 
 			if (!ShowErrorNotification || !shouldShowNotification)
 				return;