private void checkOrderNumber (Text orderNumberText) {
    String errorMessage = null;
    String orderNumber = orderNumberText.getText();
    if (orderNumber == null || !orderNumber.equals("?")){
      if (orderNumber == null || orderNumber.length() == 0){
        errorMessage = "Cislo objednavky nemoze byt prazdne!";
      } else if (!orderTableModel.getData().contains(new Order(orderNumber, null, null, null))){
        errorMessage = "Objednavka s cislom " + orderNumber + " neexistuje.";
      }
      if (errorMessage != null){
        orderNumberText.setText("?");
        MessageBox.displayMessageBox(table.getShell(), SWT.ERROR, "Chyba !", 
          errorMessage);
      }
    }
  }