Ensure that the title attribute is set before trying to add in the heading index marker
If a parser function extension inserts a heading into an article, PPFrame_DOM::expand() chokes on it because its title attribute is not set:
1041 $titleText = $this->title->getPrefixedDBkey();
Since title attributes are only required for inserting heading index markers, and these markers should most probably not be inserted for headings not within the article content, the simple fix is to check that the title attribute is set before trying to add in the heading index marker. A trivial patch is attached.