Laravel 5.5 - “Call to a member function sync() on null” Error

I'm trying to save post images. There is no problem with saving images or moving images to the uploads directory. But in sync section, it gives me that error.

(I have three tables. posts, media and pivot table)

Here is sync codes;

$media = Media::where('created_at', '>=', Carbon::now()->subSecond(10))->pluck('id');
$post->media()->sync($media, true);

Media Model;

public function posts(){
$this->belongsToMany('App\Post','media_post','image_id','post_id');

}

and Post Model

public function media() {
$this->belongsToMany('App\Media','media_post','post_id','image_id');

}

Any advice ?