ExtJS FormPanel in FormPanel fails with "this.body is null"

Question

I have a rather complicated setup, which I welded to the code below. I have an external FormPanel where I am trying to include a component that is a subclass of FormPanel . In FF, it raises the error "this.body is null".

Is this happening to someone else? Can this be made to work? I really don't want to touch a subclass if I don't need it.

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();

Answer 1

, , , xtype: 'form' xtype: 'form. , xtype: 'form', xtype: 'panel' : 'form'.

Answer 2

FormPanel FormPanel. . , :

var test = new Ext.Window({
  title: 'test',
  items: [{
    xtype: 'form',
    items: [{
        xtype: 'textfield',
        fieldLabel: 'Testing'
    }]
  }]
});
test.show();

Answer 3

, , - (, ) "".

Answer 4

Extjs HTML. .