title: Does dart null safety variable need manual release?
body: <p>I defined some nullable objects and used them elsewhere in State&lt;T&gt;</p>
  <pre class="lang-dart prettyprint-override"><code>List&lt;T&gt;? _data;
  int? _count;
  </code></pre>
  <p>Do I need to release them manually?
  like this:</p>
  <pre class="lang-dart prettyprint-override"><code>  @override
    void dispose() {
      _data?.clear(); // clear list
      _data = null;   // blank manually
      _count = null;  // blank manually
      super.dispose();
    }
  </code></pre>
  <p>Any suggestions?<br />
  Thank you!</p>