private void BtnNewInvoice_Click(object sender, RoutedEventArgs e)
 {
     try{
         clsMainLogic.Invoice newInvoice = controller.createNewInvoice();
         setCurrentInvoice(newInvoice);
     }
     catch (Exception ex) {
         HandleError(MethodInfo.GetCurrentMethod().DeclaringType.Name,
                     MethodInfo.GetCurrentMethod().Name, ex.Message);
     }
 }