How to unit test polymorphic relation laravel?

Hi everyone im trying to test my ServiceImplement. Im using polymorphic relation ..

My service its :

public function findAll($class, $classId, ApiRequest $request = null);

public function save(Comment $comment);

My ServiceImplement :

public function findAll($class, $classId, ApiRequest $request = null)
    {
        $comment = Comment::where('commentable_type', $class)->where('commentable_id', $classId)->get();

        if ($request != null) {
            $comment->load($request->getRelations());
        }

        return $comment;
    }

public function save(Comment $comment)
    {

        $model = NamespaceConstants::NAMESPACE_CONST . $comment->commentable_type;

        $rules = [
            'comment' => 'required|max:2000',
            'commentable_id' => 'required|exists:' . $model::getModel()->getTable() . ',id',
            'commentable_type' => 'required|max:255',
        ];

        $validator = \Validator::make($comment->getAttributes(), $rules);

        if ($validator->passes()) {
            $comment->timestamp = date(DateConstants::DATETIME, strtotime(date(DateConstants::DATETIME) . "+2 hours"));
            $comment->user_id = \Auth::user()->id;

            $comment->save();

            return $comment;
        }

        return \Response::json($validator->errors()->toArray(), 422);
    }

Im trying to write unit test but i have this error

ErrorException: Trying to get property of non-object

This is my code that i use to test my serviceimlpement

public function testFindAll()
    {
        $user = new User();
        $user->id = 1;
        $user->login = 'test@test.com';
        $user->password = 'password';
        $user->active = 'true';

        $userService = new UserServiceImpl();
        $userService->save($user);


        $product = new Product();
        $product->id = 1;
        $product->name = 'test';

        $productService = new ProductServiceImpl();
        $productService->save($product);

        $comment = new Comment();
        $comment->comment = 'test test';
        $comment->timestamp = date(DateConstants::DATETIME, strtotime(date(DateConstants::DATETIME) . "+2 hours"));;
        $comment->user_id = $user->id;

        // polymorphic 
        $comment->commentable_id = $product->id;
        $comment->commentable_type = 'Product';

        $commentService = new CommentServiceImpl();
        $commentService->save($comment);

        // Act
        $comment = $commentService->findAll($comment->commentable_type, $comment->commentable_id);

        // Assert
        $this->assertTrue(count($comment) > 0);

    } 

via Chebli Mohamed