code: protected function getFilterFieldValue(): string
      {
          if (isset($_SESSION[SlimPostgres::SESSION_KEY_ADMIN_LIST_VIEW_FILTER][$this->getFilterKey()][self::SESSION_FILTER_VALUE_KEY])) {
              return $_SESSION[SlimPostgres::SESSION_KEY_ADMIN_LIST_VIEW_FILTER][$this->getFilterKey()][self::SESSION_FILTER_VALUE_KEY];
          } else {
              return '';
          }
      }
docs: either session value or empty string