NullPointerException is thrown when cascade deleting a broken -to-many relationship.
Given a -to-many relationship - There are cases in which a relationship may be broken; a referenced entity is removed, but the reference to it is maintained in the owner.
 In such a case, GORM maintains the -to-many domain instance field as a persistent collection with "null" objects replacing the missing referenced entities.
  Some places, such as the cascading collection delete procedure, aren't able to handle a null object.