Prevent error when renaming outliner titles
This is an attempt to resolve #2. The walkback occurs due to
the categoryList in the pluggable outliner being empty. The
existing code attempts to get the last element and the index of
it and fails.

This fix first checks if the categoryList is empty and then stops
the editing process. The walkback no longer occurs. Unforunately
the title of the object doesn't change either. It immediately
goes back to what it was originally.
Merge pull request #18 from doublec/outliner_issue2

Prevent error when renaming outliner titles