code: protected TreeElement getTreeRoot(ExpressionHandling expr)
              throws JspException
      {
          String datasource = "{" + _dataSource + "}";
          Object treeNode = expr.evaluateExpression(datasource, "dataSource", getPageContext());
          if (treeNode == null || hasErrors()) {
              return null;
          }
  
          if (!(treeNode instanceof TreeElement)) {
              String s = Bundle.getString("Tags_TreeInvalidAttribute", _dataSource);
              registerTagError(s, null);
              return null;
          }
          return (TreeElement) treeNode;
      }
docs: Return the <code>TreeControl</code> instance for the tree control that
  we are rendering.
  @throws JspException if no TreeControl instance can be found