message: ConstObj::get_linked_object should not look up object if key is null.
diff: @@ -325,8 +325,11 @@ ConstObj ConstObj::get_linked_object(ColKey link_col_key) const
  {
  TableRef target_table = get_target_table(link_col_key);
  ObjKey key = get<ObjKey>(link_col_key);
  -
  - return target_table->get_object(key);
  + ConstObj obj;
  + if (key) {
  + obj = target_table->get_object(key);
  + }
  + return obj;
  }
  template <class T>
  