docs: public void onValueChange(ValueChangeEvent<String> event) {
  
          String message = null;
          boolean enableOk = true;
          if (m_isCreateNew) {
              if (CmsStringUtil.isEmptyOrWhitespaceOnly(m_fileName.getFormValueAsString())) {
                  enableOk = false;
                  message = Messages.get().key(Messages.GUI_EDIT_LINK_NO_FILE_NAME_0);
              } else if (CmsStringUtil.isEmptyOrWhitespaceOnly(m_linkContent.getFormValueAsString())) {
                  enableOk = false;
                  message = Messages.get().key(Messages.GUI_EDIT_LINK_NO_LINK_0);
              }
          } else {
              if ((m_linkContent.getFormValueAsString().equals(m_previousLink)
                  && (m_linkTitle.getFormValueAsString().equals(m_previousTitle)))) {
                  enableOk = false;
                  message = Messages.get().key(Messages.GUI_EDIT_LINK_NO_CHANGES_0);
              } else if (CmsStringUtil.isEmptyOrWhitespaceOnly(m_linkContent.getFormValueAsString())) {
                  enableOk = false;
                  message = Messages.get().key(Messages.GUI_EDIT_LINK_NO_LINK_0);
              }
          }
          setOkEnabled(enableOk, message);
      }