How to test Laravel api resource

How can I test the api resource without hitting the database ? , I tried to do the following, but it throws an error.

        $user = new User([
            'username' => 'Jonh',
            'email' => '[email protected]',
            'password' => bcrypt('secret'),
            'created_at' => Carbon::now(),
            'updated_at' => Carbon::now(),
        ]);
        Topic::unguard();
        $user->setRelation('topics', [
            new Topic([
                'id' => 1,
                'section_id' => 1,
                'user_id' => 1,
                'body' => 'Test post',
                'title' => 'something',
                'slug' => 'donoqwdnowq',
                'created_at' => Carbon::now(),
                'updated_at' => Carbon::now(),
            ])
        ]);
        $resource = (new UserResource($user))->toArray(request());

        $this->assertEquals(['username','email','avatar','topics'], array_keys($resource));

Here is the UserResource

    public function toArray($request) {
        return [
            'username' => $this->username,
            'email' => $this->email,
            'avatar' => $this->avatar(),
            'topics' => new TopicResource($this->whenLoaded('topics'))
        ];
    }

and Topic Resource

    public function toArray($request) {
        return [
            'title' => $this->title,
            'slug' => $this->slug,
            'body' => $this->body,
            'diffForHumans' => $this->created_at->diffForHumans(),
            'user' => new UserResource($this->user),
            'section' => new SectionResource($this->whenLoaded('section')),
            'comments' => CommentResource::collection($this->whenLoaded('comments')),
        ];
    }

Error :

E                                                                   1 / 1 (100%)

Time: 2.44 seconds, Memory: 18.00MB

There was 1 error:

1) Tests\Unit\Users\Domain\Resources\UserResourceTest::it_returns_posts_associated_with_user_on_loading
ErrorException: Trying to get property 'title' of non-object

/Users/mohammed/projects/cross-api/vendor/laravel/framework/src/Illuminate/Http/Resources/DelegatesToResource.php:120
/Users/mohammed/projects/cross-api/app/Forum/Domain/Resources/TopicResource.php:19
/Users/mohammed/projects/cross-api/tests/Unit/Users/Domain/Resources/UserResourceTest.php:54

Note: it worked well when I wasn't passing something to load like topics, I think there is just a problem with loading nested resources but I don't know how to tweak it around.