I get this error when I insert a new post in the db "Call to a member function load() on null"

When I try to add a new post I get the error

Call to a member function load() on null

and this is my PostController:

public function index($id = NULL)
    $posts = Post::all();

    $user = User::find($id)->load(['posts']);     
    return view('post.index', compact('posts','user'));
}

TABLES:

countries
    id - integer
    name - string

users
    id - integer
    country_id - integer
    name - string

posts
    id - integer
    user_id - integer
    title - string

How can I fix it?