docs: public void addSubFileFilter()
      {
          // Override this if it is not correct.
          SubFileFilter listener = null;
          this.getMainRecord().addListener(listener = new SubFileFilter(this.getHeaderRecord()));
          if (!this.getMainRecord().getKeyArea().getField(DBConstants.MAIN_KEY_FIELD).isNullable())
              listener.setFilterIfNull(true); // If the header record's key can't be null, don't display any detail if new record
      }