BUG: check that the frame image has been initialized

Fixes #3535


git-svn-id: http://svn.slicer.org/Slicer4/trunk@22786 3bd1e089-480b-0410-8dfb-8563597acbee
ENH: Add 'DataStore' to top-level module category.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22787 3bd1e089-480b-0410-8dfb-8563597acbee