code: private void addExtensionPoint(final Element elem) {
          final String id = elem.getAttribute(EXTENSION_POINT_ID_ATTR);
          if (id == null) {
              throw new IllegalArgumentException(EXTENSION_POINT_ID_ATTR + " attribute not set on extension-point");
          }
          final String name = elem.getAttribute(EXTENSION_POINT_NAME_ATTR);
          features.addExtensionPoint(new ExtensionPoint(id, name, currentPlugin));
      }
docs: Add extension point.
  
  @param elem extension point element attributes
  @throws IllegalArgumentException if extension ID is {@code null}