private void deleteButton_Click(object sender, RoutedEventArgs e)
 {
     if (supplierTabControl.IsSelected)
     {
         if (selectedSupplier != null)
         {
             foreach (ISupplier supplier in suppliersDataGrid.SelectedItems)
             {
                 supplierController.DeleteSupplier(supplier);
             }
             suppliersDataGrid.SelectedItem = null;
             refreshDataGrid();
         }
     }
     else if (paymentRuleTabControl.IsSelected)
     {
         if (selectedPaymentRule != null)
         {
             foreach (IPaymentRule paymentRule in paymentRuleDataGrid.SelectedItems)
             {
                 supplierController.DeletePaymentRule(paymentRule);
             }
             paymentRuleDataGrid.SelectedItem = null;
             refreshPaymentRuleDataGrid();
         }
     }
 }