title: Issue after upgrade flutter
body: <p>Hello I have this issue after upgrade flutter ... </p>
  
  <pre><code>'package:flutter/src/rendering/sliver_multi_box_adaptor.dart': Failed assertion: line 263 pos 16: 'child == null || indexOf(child) &gt; index': is not true.
  </code></pre>
  
  <p>I havn't this issue before.
  Any idea ? thank you </p>