Fixed incorrect use of DOM.insertBefore API.

* When I wanted to add an element in the first child position,
  I incorrectly passed 'null' which added it to the last child
  position which then introduces an infinite loop when processing
  mw:DiffMarkers in certain scenarios.

Change-Id: I986f8e92ecd762f6f5a9c284b87ba6daa0f13fc2