protected function rowFiltersJsonOrNull()
 {
     if (empty($this->rowFilters)) {
         return null;
     } else {
         if (count($this->rowFilters) === 1) {
             return $this->rowFilters[0]->toJsonStr();
         } else {
             $filterGroup = new FilterGroup($this->rowFilters);
             return $filterGroup->toJsonStr();
         }
     }
 }