Call to a member function id() on null in convert_gantt_data()

The code causing that error is the following one.

          foreach ($values as $value) {
            if (!empty($value->entity)) {
              $data[$group][$delta][$ganttField][] = $value->entity->id();
            }
          }
          // @todo remove later for multiuser.
          $data[$group][$delta][$ganttField] = $value->entity->id();

The last line accesses $value->entity->id() without first checking $value->entity isn't empty, like the other line does.