title: Call to a member function lists() on null
body: <p>When I want to edit a post I'm getting an error.
  This is the code from the line where the error is:</p>
  
  <pre><code>public function getTagListAttribute()
  {
      return $this-&gt;tags-&gt;lists('id')-&gt;all();
  }
  </code></pre>
  
  <p>It works perfectly when I read the full post but when I want to edit it I get thrown this error: <code>Call to a member function lists() on null</code></p>
  
  <p>I have also categories code:</p>
  
  <pre><code>public function getCategoryListAttribute()
  {
      return $this-&gt;categories-&gt;lists('id')-&gt;all();
  }
  </code></pre>
  
  <p>And I can edit fine categories but not tags, and they're made the same principal.</p>