public static void ShowSingleWindow()
 {
     if (Window == null)
     {
         Window = new NewKupciWindow {
             Owner = Application.Current.MainWindow
         };
         Window.Show();
     }
     else
     {
         Window.Activate();
     }
 }
 private static void NewKupciWindow_Closing(object sender, System.ComponentModel.CancelEventArgs e)
 {
     Window = null;
 }