Strict warning: Only variables should be passed by reference in render()

Drupal's render() function passed the argument by reference, so we can't use the result of the menu_tree() function directly, here:

function render($empty = FALSE) {
    if (!empty($this->options['menu'])) {
      return render(menu_tree($this->options['menu']));
    }
    return '';
  }

This should be like:

function render($empty = FALSE) {
    if (!empty($this->options['menu'])) {
      $tree = menu_tree($this->options['menu']);
      return render($tree);
    }
    return '';
  }

Provided a patch with a proposed solution.