docs: private String getMessage(Throwable t) {
  
          if ((t instanceof I_CmsThrowable) && (((I_CmsThrowable)t).getMessageContainer() != null)) {
              StringBuffer result = new StringBuffer(256);
              if (t instanceof CmsMultiException) {
                  CmsMultiException exc = (CmsMultiException)t;
                  String message = exc.getMessage(getLocale());
                  if (CmsStringUtil.isNotEmpty(message)) {
                      result.append(message);
                      result.append("<br />");
                  }
  
              }
  
              I_CmsThrowable cmsThrowable = (I_CmsThrowable)t;
              result.append(cmsThrowable.getLocalizedMessage(getLocale()));
              return result.toString();
          } else {
              String message = t.getMessage();
              if (CmsStringUtil.isEmptyOrWhitespaceOnly(message)) {
                  // no error message found (e.g. for NPE), provide default message text
                  message = CmsVaadinUtils.getMessageText(Messages.GUI_ERROR_UNKNOWN_0);
              }
              return message;
          }
      }