BUG: check for null id

Temporary fix for the BrainAtlas scene view restore bug 
http://na-mic.org/Mantis/view.php?id=2557

Issue #2557



git-svn-id: http://svn.slicer.org/Slicer4/trunk@21843 3bd1e089-480b-0410-8dfb-8563597acbee