ModalPopupExtender作为状态框

我有一个ModalPopupExtender，我只是想使用作为一个状态弹出时，有一个长时间运行的后台任务。

<script language="javascript" type="text/javascript">
    function ShowInfoPopup(Caption, Message) {
        $find("<%= tcPopUpCaption.ClientID %>").innerHTML = Caption;
        $find("<%= tcPopUpMessage.ClientID %>").innerHTML = Message;
        $find("<%= mpe.ClientID %>").Show();
    }
</script>
<asp:LinkButton runat="server" ID="btnPopup" />
<asp:Panel ID="pnlInfoPopup" runat="server" Style="display: none;"
           Width="350px" CssClass="InfoPopup_Panel">
    <asp:Table ID="Table1" runat="server" CssClass="InfoPopup_Layout"
               Style="margin-bottom: 0px">
        <asp:TableRow CssClass="InfoPopup_ActionRow">
            <asp:TableCell BorderStyle="None">
                <asp:Button ID="btnClose" runat="server" Text="x"
                            ToolTip="Close" BorderStyle="Outset"
                            UseSubmitBehavior="false" OnCommand="doCommand"
                            CommandName="ClosePopUp" />
            </asp:TableCell>
        </asp:TableRow>
        <asp:TableHeaderRow Style="border-bottom: thick solid white">
        <asp:TableHeaderCell ID="tcPopUpCaption" Width="100%"
                                  BorderStyle="None" runat="server"
                                  Text="Popup Caption" />
        </asp:TableHeaderRow>
        <asp:TableRow>
        <asp:TableCell Width="50%" BorderStyle="None" runat="server"
                            ID="tcPopUpMessage" ClientIDMode="Static"
                            Text="Popup Message" />
        </asp:TableRow>
    </asp:Table>
</asp:Panel>
<asp:ModalPopupExtender ID="mpe" runat="server" PopupControlID="pnlInfoPopup"
                        BackgroundCssClass="mpeBackground" DropShadow="true"
                        TargetControlID="btnPopup" />
<asp:Button ID="btnS" runat="server" Text="Send" OnCommand="doCommand"
            CommandName="send" UseSubmitBehavior="false" Width="100px"
            Height="45px" ClientIDMode="Static"
            OnClientClick="ShowInfoPopup('Please wait...', 'Sending...')" />
<asp:Button ID="btnL" runat="server" Text="Load" OnCommand="doCommand"
            CommandName="load" UseSubmitBehavior="false" Width="100px"
            Height="45px" ClientIDMode="Static"
            OnClientClick="ShowInfoPopup('Just a sec...', 'Loading...')" />

当用户点击其中一个按钮时(这只是一个示例场景)，它会显示带有相关标题和消息的弹出框，然后执行正常的Postback来执行(长)任务。

然而，在"ShowInfoPopup"脚本中，$find的标题和消息失败，因为它找不到控件，它返回NULL。

欢迎有任何建议