title: Error Uncaught TypeError: Can not read property 'addCls' of null
body: <p>I have an application that works perfectly but in the console I get the following error: </p>
  
  <p>Uncaught TypeError: Can not read property 'addCls' of null </p>
  
  <p>But the application works correctly, because I sent this error if the application works?</p>
  
  <hr>
  
  <p>I found the problem, but not how to fix it.<br>
  in my grid when I add: </p>
  
  <pre><code>features: [{ftype: 'grouping'}]
  </code></pre>
  
  <p>I start to get this error. </p>
  
  <p>hopefully you can help me. 
  regards</p>