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);
      }
    }
  }