Class 'app\controllers\Html' not found

I am beginner in Yii 2.x. I have successfully completed CURD manually. Everything working fine. While I am adding flash message I got this error message.

PHP Fatal Error – yii\base\ErrorException

Class 'app\controllers\Html' not found

I have added use app\controllers\Html; in my controller still no changes there.

Wrong namespace?

Thanks @PatrickJones