modalpopup getting dissappear when press enter botton.

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ASP.NET

Hi,
I have a gridview when I click on grid view row a modal popup is getting open.
I have wriiten code to open popup in codebehind as
this.ModalPopupExtender1.Show();
my popup div as follow

<div>
    <div>
        <asp:Button ID="Button1" runat="server" Style="display: none" />
        <cc1:ModalPopupExtender ID="ModalPopupExtender2"  runat="server" TargetControlID="btnShowPopup"
            PopupControlID="pnlpopup" CancelControlID="btnCancel" BackgroundCssClass="modalBackground">
        </cc1:ModalPopupExtender>
        <asp:Panel ID="Panel1" runat="server" BackColor="White" Style="display: none">
          
            <div style="width: 400px; background-color: White; padding: 20px; border: Solid 1px gray;
                box-shadow: 10px 10px 10px #5d5d5d">
                <table style="border: Solid 0px gray; background-color: #EAEAEA; width: 400px; height: 350px;"
                    cellpadding="0" cellspacing="0" border="0">
                    <tr>
                        <td class="normaltd" nowrap="nowrap" style="width: 100px; text-align: left; padding-left: 10px;
                            padding-top: 5px;">
                        Company
                        </td>
                        <td nowrap="nowrap" style="width: 300px; text-align: left;" valign="top">
                            <input id="Hidden1"  runat="server" type="hidden" />
                            <asp:ComboBox ID="ComboBox1" runat="server" CssClass="comboInsideModal" AutoCompleteMode="Suggest"
                                Width="250" ValidationGroup="S">
                            </asp:ComboBox>
                        </td>
                    </tr>
                </table>
            </div>
        </asp:Panel>
    </div>

</div>

The popup div contains a ajax combobox editable dropdown.
Whenever user press enter to select item from the combobox ,the modalpopup gets dissapear. How to solved this problem ?