That looks like a different error that occurs in org-ref-open-bibtex-notes(). Up to that point everything looks normal.
What do you have your org-ref-bibliography-notes variable set to? The default value is nil which would cause that error.
John
----------------------------------- Professor John Kitchin Doherty Hall A207F Department of Chemical Engineering Carnegie Mellon University Pittsburgh, PA 15213 412-268-7803