code: public static function setInstance($instance)
      {
          if ($instance === null) {
              unset(Yii::$app->loadedModules[get_called_class()]);
          } else {
              Yii::$app->loadedModules[get_class($instance)] = $instance;
          }
      }
docs: Sets the currently requested instance of this module class.
  @param Module|null $instance the currently requested instance of this module class.
  If it is `null`, the instance of the calling class will be removed, if any.