code: public function getStandaloneViewers() {
          $ret = [];
          foreach ($this->getValueViewers() as $key => $viewer) {
              if (!$viewer->isLinkedToDbColumn()) {
                  $ret[$key] = $viewer;
              }
          }
          return $ret;
      }
docs: Get only viewers that are not linked to columns defined in TableConfig class ($valueViewer->isDbColumn() === false)
  @return AbstractValueViewer[]|DataGridColumn[]|ValueCell[]|FormInput[]