var viewport = new Ext.Viewport({
      items:[ {
        xtype:"tabpanel",
        id:"tpanel",
        baseCls:"x-plain",
        activeTab:0,
        listeners:{
          "render":function(){
              var p = this.add(new Ext.Panel({
                     id: 'id',
                     html:"hello"
                 })).show();
             }
           }
      }]
  });