Fix for #35147, view in backdrop and image editor in compositor don't work anymore. The active_viewer_key which sets the active node tree to use for the viewer image was not initialized in do_versions yet.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@56408 954f8c5b-7b00-dc11-b283-0030488c597c