Hi
Try this code:-
<div id="GlobalMain" runat="server">
<asp:Button ID="btndummy" CssClass="hiddencol" runat="server" Text="dummy" OnClick="btndummy_Click" /></div>
<asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server" EnablePageMethods="true">
</asp:ToolkitScriptManager>
<asp:ModalPopupExtender ID="GlobalModalPopupExtender" runat="server" PopupControlID="GlobalpnlPopup"
DropShadow="False" TargetControlID="GlobalMain" BackgroundCssClass="modalBackground">
</asp:ModalPopupExtender>
<asp:Panel ID="GlobalpnlPopup" CssClass="modalPopup" Style="display: none;" Width="550px"
runat="server">
<asp:UpdatePanel ID="UpdatePanel2" runat="server" UpdateMode="Always">
<ContentTemplate>
<table style="width: 550px; font-size: larger">
<tr>
<td align="center">
<div class="tmtable-main-container">
<div class="tmtable-title">
<div class="tmtable-title-text">
Existing Case
</div>
</div>
</div>
</td>
</tr>
<tr>
<td id="Td1" class="textbg" runat="server">
<asp:RadioButtonList ID="RadioButtonList1" runat="server">
</asp:RadioButtonList>
</td>
</tr>
<tr>
<td id="Td2" align="center" class="textbg" runat="server">
<asp:Button ID="btnCaseSubmit" runat="server" Text="Submit" OnClick="btnCaseSubmit_Click"
Width="76px" />
<asp:Button ID="btnClose" runat="server" Text="Close" Width="76px" />
</td>
</tr>
</table>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="btnCaseSubmit" EventName="Click" />
<asp:AsyncPostBackTrigger ControlID="btnClose" EventName="Click" />
</Triggers>
</asp:UpdatePanel>
</asp:Panel>
Please mark if it works for you