code: public function actionIndex(string $page = null): string
      {
          if (!$page) {
              $page = $this->getPageName();
          }
  
          $page = $this->module->find($page);
  
          if ($page === null) {
              throw new NotFoundHttpException(Yii::t('yii', 'Page not found.'));
          }
  
          return $this->renderPage($page);
      }
docs: Index action.
  @param string|null $page
  @return string rendered page
  @throws NotFoundHttpException
  @throws \yii\base\InvalidConfigException