自定义用户控件内的ASP.NET Ajax模式弹出扩展程序显示问题

我遇到的问题是,当试图显示模式弹出窗口以便用户可以与其交互时,根本没有显示任何内容,代码如下:

用户控件:

<asp:UpdatePanel ID="breakTheGlassUpdatePanel" runat="server">
<ContentTemplate>
    //some fields for entry and a submit/cancel button
</ContentTemplate>
</asp:UpdatePanel>

<ajaxToolkit:ModalPopupExtender runat="server" ID="breakModal"
    TargetControlID="breakModalHolder" 
    BackgroundCssClass="btgModalBackground" 
    DropShadow="true" 
    PopupControlID="breakTheGlassUpdatePanel" />

    <asp:Button ID="breakModalHolder" runat="server" Style="display:none;" />

父页面中的代码:

BreakTheGlassControl btgControl = (BreakTheGlassControl)Page.LoadControl("~/ARISUserControls/BreakTheGlassControl.ascx");
AjaxControlToolkit.ModalPopupExtender breakModal = (AjaxControlToolkit.ModalPopupExtender)btgControl.FindControl("breakModal");
breakTheGlassPanel.Controls.Add(btgControl);//panel in parent page
breakTheGlassPanel.Visible = true;
breakTheGlassPanel.Controls.Add(btgControl);
breakModal.Show();