protected function getHtmlErrorCell(): string
  {
    $ret = '';

    if (!$this->isValid())
    {
      $error_messages = $this->getErrorMessages(true);

      $ret .= '<td class="error">';
      if (!empty($error_messages))
      {
        foreach ($error_messages as $message)
        {
          $ret .= Html::txt2Html($message);
          $ret .= '<br/>';
        }
      }
      $ret .= '</td>';
    }
    else
    {
      $ret .= '<td class="error"></td>';
    }

    return $ret;
  }