`BKE_collection_object_cache_free` can be called from some points in
code (e.g. during ID remapping) where proper parenting relationships
between collections cannot be ensured.
Would be good to check if recursive cache cleanup is actually needed
from these points, it would almost certainly be better and safer to
instead tag parent collections as dirty too, but this will be for
another time.
For now, just add a NULL-check on the parent collection pointer.
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