'this._postBackSettings.async' is null

 okay, the error 'this._postBackSettings.async' is null  is thrown when the below script is executed in what I am assuming is script from the ScriptManager.

// If the postback happened from outside an update panel, fall back
// and do a normal postback.
if (!this._postBackSettings.async) {
    return;
}

this is at the top of the scriptResource:

// Name:        MicrosoftAjaxWebForms.debug.js
// Assembly:    System.Web.Extensions
// Version:     3.5.0.0
// FileVersion: 3.5.21022.8
//!-----------------------------------------------------------------------
//! Copyright (C) Microsoft Corporation. All rights reserved.
//!-----------------------------------------------------------------------
//! MicrosoftAjaxWebForms.js
//! Microsoft AJAX ASP.NET WebForms Framework.

// Partial Rendering

  what I have is a simple table cell with a link button that popups up a panel using the PopupControlExtender: (code snippet)

<td align="left" valign="top"  style="width: 100px"><asp:LinkButton ID="btnBrandFilter" runat="server" Text="Brand"        CssClass="linkButton" CausesValidation="False"        ToolTip="Click here to set your Brand filter criteria"/>   <asp:Panel ID="pnlBrandPopup" runat="server" CssClass="popupControl"  Width="200"><asp:CheckBoxList  ID="BrandPopupCheckBoxList" runat="server" AutoPostBack="false" /><br /><asp:Button ID="btnBrandPopupOk" runat="server" Text="ok"  OnClick="btnBrandPopupOk_Click" /><asp:Button ID="btnBrandPopupCancel" runat="server" Text="cancel" /><input type="button" onclick="checkall('brand')" value="de-Select all" id="btnChkallBrand" /></asp:Panel><cc1:PopupControlExtender  ID="PopupControlExtenderBrands" runat="server" TargetControlID="btnBrandFilter" PopupControlID="pnlBrandPopup"  Position="Bottom" /></td>

everytime I click a button in the panel, I am getting that script error.  Is this a bug?  and has anyone found a solution or a VALID work around? (note I said VALID, going into the scriptManager and setting EnablePartialRendering to false is not valid.)

I've searched this forum and googled for other answers, and I can't find a valid fix.  Lot's of people have had the same problem, but no where have I seen a valid solution. 

thanks in advance if you have a solution.