docs: @Override
      public DefaultMutableTreeNode getDataAsTree() {
          HelpSet hs = getHelpSet();
          Hashtable<?, ?> params = getParameters();
          URL url;
  
          if (params == null || !params.containsKey("data")) {
              return new DefaultMutableTreeNode();
          }
  
          try {
              url = new URL(hs.getHelpSetURL(), (String) params.get("data"));
          } catch (Exception ex) {
              throw new Error("Trouble getting URL to TOC data; " + ex);
          }
  
          return parse(url, hs, hs.getLocale(), new TreeItemFactoryImpl(), this);
      }