[CLOSED] Grouptabpanel in window prevents window from rendering

[SymSure]

Hi, window that contains group tab panel does not render at all.
I have a window with the following code snippet

<%@ Control Language="C#" Inherits="System.Web.Mvc.ViewUserControl<dynamic>" %>
<%@ Register Assembly="Ext.Net" Namespace="Ext.Net" TagPrefix="ext" %>
<ext:Window ID="winNewResultSet" runat="server" Title="Model.Title" AutoDataBind="true"
    Width="600" Height="500" Border="false" Closable="true" Draggable="true" Modal="true"
    CloseAction="Destroy" IDMode="Static" Resizable="false" Layout="FitLayout">
    <Items>
        <ext:GroupTabPanel ID="grouptabMainNewResultSet" runat="server"  Padding="3" Margins="5">
            <Items>
                <ext:Panel runat="server" Title="Test 1">
                    <Items>
                        <ext:Panel Title="Hello world" runat="server"></ext:Panel>
                    </Items>
                </ext:Panel>
                 <ext:Panel  runat="server" Title="Test 2">
                
                </ext:Panel>
            </Items>
            
            
        </ext:GroupTabPanel>
    </Items>    
</ext:Window>

when I replace the grouptabpanel with a normal panel, the window is rendered. But otherwise the window does not render. How can I make the grouptabpanel work in a window.