Cause
This error occurs if the schema file defines a reference as being an exclusive abstract collection type.
Exclusive references are always created when the parent object is created, so they must have a real (non-abstract) type.
Action
Change the type of the reference to a real (non-abstract) collection. This is likely to be a concrete subclass of the original abstract collection.
Alternatively, make the reference to the abstract collection a shared reference rather than an exclusive reference.