private void BtnDeleteInvoice_Click(object sender, RoutedEventArgs e)
 {
     try{
         controller.deleteInvoice(currentInvoice);
         updateAllInvoices();
         clearDisplay();
     }
     catch (Exception ex) {
         HandleError(MethodInfo.GetCurrentMethod().DeclaringType.Name,
                     MethodInfo.GetCurrentMethod().Name, ex.Message);
     }
 }