code: public function createCaptchaAction()
      {
          $ca = Yii::$app->createController($this->captchaAction);
          if ($ca !== false) {
              /* @var $controller \yii\base\Controller */
              list($controller, $actionID) = $ca;
              $action = $controller->createAction($actionID);
              if ($action !== null) {
                  return $action;
              }
          }
          throw new InvalidConfigException('Invalid CAPTCHA action ID: ' . $this->captchaAction);
      }
docs: Creates the CAPTCHA action object from the route specified by [[captchaAction]].
  @return \yii\captcha\CaptchaAction the action object
  @throws InvalidConfigException