Refer to http://www.avaje.org/topic-172.html

public Set<Usergroup> getGroups() {
    if ( groups == null ) {
      final BeanSet<Usergroup> set = new BeanSet<Usergroup>();
      set.setModifyListening(BeanCollection.ModifyListenMode.ALL);
      setGroups(set);
    }
    return groups;
  }

Consider changing the enhancer to automatically add the "null protection" type code for *ToMany properties.