Nice to have
In case of an invalid migration, data consistency may be lost, so that object identifiers stored in foreign key columns (tl:ByIDReferenceStorageImpl) can no longer be resolved. In this case, accessing such an invalidated reference silently returns the value null, which makes debugging extremely difficult.
Instead, as with resolving an association whose end cannot be found, a tl:InvalidLinkException should be thrown.
No test, too complex.