title: extjs 4 dynamically enable an actioncolumn item
body: <p>Using 4.0.6 I have a gridpanel with an actioncolumn that has two items, a delete button and and view button. The delete button is disabled but there are some circumstances where it needs to be enabled (if a user is an admin). So I am trying to enable it in code.</p>
  
  <p>Here's the view:</p>
  
  <pre><code>Ext.define('PP.view.person.List', {
  extend: 'Ext.grid.Panel',
  alias: 'widget.personlist', 
  title: 'Current people',
  store: 'People',
  columnLines: true,
  
  initComponent: function () {
      this.columns = [
          { header: 'Id', dataIndex: 'personId', flex: 1, sortable: true },
          { header: 'Title', dataIndex: 'title', flex: 1 },
          { header: 'Name', dataIndex: 'name', flex: 1},
          { xtype:'actioncolumn',
              items:[
                  {icon:'cross.gif',disabled:true,handler:function(){alert('delete pressed');}},
                  {icon:'tick.gif',handler:function(){alert('view pressed');}}
              ]
          }
      ];
  
      this.callParent(arguments);
  }
  </code></pre>
  
  <p>});</p>
  
  <p>and the controller function I'm using to enable:</p>
  
  <pre><code>enableDel: function () {
      var view = Ext.widget('personlist');
      view.down('actioncolumn').enableAction(0);
  }
  </code></pre>
  
  <p>but I am getting an error inside the extjs enableAction function on the line that starts</p>
  
  <pre><code>me.up('tablepanel').el.select
  </code></pre>
  
  <p>the el is undefined. The column doesn't get enabled either. Can anyone tell me what I'm doing wrong or post an example that works? If I specify</p>
  
  <pre><code>renderTo: Ext.getBody()
  </code></pre>
  
  <p>in the view I don't get the error any more but the column still doesn't enable. So I think it must be something to do with rendering but I'm so new to this I haven't a clue where to start.</p>