title: ExtJS FormPanel in a FormPanel fails with "this.body is null"
body: <p>I have a rather complicated setup which I have boiled down to the code below.  I have an outer <strong>FormPanel</strong>, where I am trying to include a component that is a sub-class <strong>FormPanel</strong>.  In FF it is causing a "this.body is null" error.</p>
  
  <p>Is this happening to anyone else?  Is it possible to get this to work?  I very much do not want to have to touch the subclass if I don't have to.</p>
  
  <pre><code>var test = new Ext.Window({
    title: 'test',
    items: [{
      xtype: 'form',
      items: [{
        // this is where the subclass of FormPanel goes
        xtype: 'form',
        items: [{
          xtype: 'textfield',
          fieldLabel: 'Testing'
        }]
      }]
    }]
  });
  test.show();
  </code></pre>