Modal popup extender not displaying popup - asp.net

Question

I have inherited some code using an ASP modal popup extender and for some reason it has now decided to stop working. Am I missing anything here? I'm not too fluent with AJAX / ASP.Net right now so im struggling to diagnose what is going wrong here.
So firstly the javascript method that displays the modal
function ShowModalRemoveEvent(ID, EventID, InstructorID, InstructorName)
{
alert("Hi");
var modal = $find('ModalPopupExtender1');
modal.show();
document.getElementById("txtID").value = ID;
document.getElementById("txtEventID").value = EventID;
document.getElementById("txtInstructorID").value = InstructorID;
document.getElementById("txtInstructorName").value = InstructorName;
}
I know this is being called because of the alert. So I would assume the prpblem is either in finding the ModalPopupExtender1 or the modal.show() line.
Here is the code for my modal popup extender
<asp:UpdatePanel ID="UpdatePanel4" runat="server" UpdateMode="Conditional">
<ContentTemplate>
<asp:Panel ID="Panel4" runat="server" Style="display: none" CssClass="ModalBackground">
<table style="width: 100%" border="0" cellpadding="0" cellspacing="0">
<tr>
<td width="7%" class="cellAlertHeader">
<img src="../../../images/Ops/window_delete.png" style="width: 18px; height: 18px" alt=""/>
</td>
<td width="93%" class="cellAlertHeader">
<b>Cancel Instructor Event</b>
</td>
</tr>
</table>
<table width="100%">
<tr>
<td width="5%">
<td width="95%">
<asp:TextBox ID="txtID" Style="display: none" runat="server" CssClass="TextboxTitle"></asp:TextBox>
<asp:TextBox ID="txtEventID" Style="display: none" runat="server" CssClass="TextboxTitle"></asp:TextBox>
</td>
</tr>
<tr>
<td width="5%">
</td>
<td width="95%">
<asp:TextBox ID="txtInstructorID" Style="display: none" runat="server" BorderStyle="None"
CssClass="TextboxTitle"></asp:TextBox>
<asp:TextBox ID="txtInstructorName" Style="font-size: 12px" runat="server" BorderStyle="None"
CssClass="TextboxTitle" BorderWidth="0"></asp:TextBox>
</td>
</tr>
<tr>
<td width="5%">
</td>
<td width="95%">
</td>
</tr>
<tr>
<td width="5%">
</td>
<td width="95%">
<asp:Label ID="Label2" runat="server" Text="Are you sure you want to remove this Event?"
CssClass="TextboxTitle"></asp:Label>
</td>
</tr>
<tr>
<td width="5%">
</td>
<td width="95%">
</td>
</tr>
<tr>
<td width="5%">
</td>
<td width="95%">
<asp:Button ID="ButtonConfirmRemove" runat="server" Text="Confirm" CssClass="buttonshort" />
<asp:Button ID="ButtonCancel" runat="server" Text="Cancel" CssClass="buttonshort" />
</td>
</tr>
<tr>
<td width="5%">
</td>
<td width="95%">
</td>
</tr>
</table>
</asp:Panel>
<asp:Button runat="server" ID="dummyButton" Style="display: none" />
<cc1:ModalPopupExtender ID="ModalPopupExtender1"
runat="server"
PopupControlID="Panel4"
TargetControlID="dummyButton"
BackgroundCssClass="ModalBGOpacity"
CancelControlID="ButtonCancel"
OnCancelScript="HideModalPopup()">
</cc1:ModalPopupExtender>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="dummyButton" />
</Triggers>
</asp:UpdatePanel>

Answer 1

Try getting the rendered id of your control, like that.
var modal = $find('<%=ModalPopupExtender1.CliendID%>');