Call to a member function delete() on null

I want to delete a row from table. The function I used in controller is:

public function destroy($id)
{
    $post = Post::find($id);
    $post->delete();
    Session::flash('success', 'The post was just trashed.');

    return redirect()->back();
}

However it throws me an error:

Call to a member function delete() on null

via Chebli Mohamed