BUG: fix loading of derived items

A typo in a python call was masked by the catching
off an attribute error meant to catch if there
were no derived items. Fixed the call to processEvents
and now derived items will load.

Issue 3806

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23737 3bd1e089-480b-0410-8dfb-8563597acbee