code: public WffBMData getWffBMData(final AbstractHtml tag, final String key)
              throws InvalidTagException, NullValueException {
  
          if (tag == null) {
              throw new NullValueException("tag cannot be null");
          }
          if (tag instanceof NoTag) {
              throw new InvalidTagException(
                      "NoTag and Blank tag are not allowed to use");
          }
          if (key == null) {
              throw new NullValueException("key cannot be null");
          }
          if (!browserPage.contains(tag)) {
              throw new InvalidTagException(
                      "The given tag is not available in the corresponding browserPage instance.");
          }
          return AbstractHtmlRepository.getWffData(tag, key);
      }
docs: gets the WffBMObject/WffBMArray from the {@code wffObjects} property of
  tag in the form of WffBMData.
  
  @param tag
  @param key
  @return WffBMData which may be type casted to either WffBMObject or
  WffBMArray.
  @throws InvalidTagException
  if the given instance is of NoTag / Blank
  @throws NullValueException
  if tag or key is null
  @since 3.0.1
  @author WFF