Add instance.id check to get_list() calls in backend editor

Add instance.id check to get_list() calls in backend editor

Some get_list() implementations for backend editors did not properly
check that the instance they were passed actually existed, and could
crash if it didn't (most prominent example being the the
sessions<->slides link). Some of these are can't-happen (because the
object doesn't support adding new empty ones), but this commit adds a
guard to those as well just to be on the safe sid for future changes.