Yii2: Class 'ActiveForm' not found?

Question

Yii

So the problem is this:
There is an action in siteController

public function actionProfile()
    {
        if (Yii::$app->user->isGuest) {
            return $this->goHome();
        }
        $model = new ProfileForm();
        $fields = $model->loadProfileData($language);
        return $this->render('user-profile', array(
            'fields'=>$fields,
            'model'=>$model,
        ));
    }

Everything works great here.
Next, there is an additional controller serving the user profile page, in which there is an action save, which is called via an ajax request (when the save button is clicked) and renders the same view as the action in the siteController.
However, when rendering I get an error Class 'ActiveForm' not found?
Can anyone suggest why this is happening?
Here is the additional controller code:

class ProfileController extends Controller {

 public function actionSave()
    {
         if (Yii::$app->user->isGuest) {
            return $this->goHome();
        }
        $request = Yii::$app->getRequest();
        $model = new ProfileForm();
        if ($request->isPost && $model->load($request->post())) {
            $fields = $model->loadProfileData($language);
            $message = "Your profile data has been saved";
            return $this->render('user-profile', array(
                    'fields' => $fields,
                    'model' => $model,
                    'message' => $message
            ));
        }
    }

Answer 1

and renders the same view as the action in the siteController.

No, not the same one. The ProfileController renders a view located at @app/views/profile/user-profile.php