When click print button(inside panel) modalpop extender automatically hide

<pre><asp:Button ID="btnGetSelected" runat="server" Text="Get selected records" OnClick="btnGetSelected_Click" />
         <asp:CustomValidator ID="CustomValidator1" runat="server" Style=" font-size:x-large" ErrorMessage="Please select at least one record." 
    ClientValidationFunction="Validate" ForeColor="Red"></asp:CustomValidator>
      
        <asp:Button ID="ButtonHdn" runat="server" Text="Button" Style="display: none;"  />
        </center>
          
        <cc1:ModalPopupExtender ID="modalpop1" runat="server" PopupControlID="Panel1" TargetControlID="ButtonHdn"  dynamicservicepath=""
                enabled="True"
                 BackgroundCssClass="modalBackground">
        </cc1:ModalPopupExtender>
        <asp:Panel ID="Panel1" runat="server" CssClass="modalPopup" align="center" style="max-height: 500px; overflow: auto; display:none; ">
               <asp:Button ID="btnClose" runat="server" Text="Close" OnClick="btnClose_Click" />
          <asp:Button ID="Btbprint" runat="server" Text="Print" OnClientClick="printGrid()" />
          
                <table id="table1" runat ="server" >
                <tr>
                <td>
             <asp:Literal ID="Literal1" runat="server"></asp:Literal>
                     </td>
                </tr>
                    
                    </table>
     
</asp:Panel>
        </ContentTemplate>
    </asp:UpdatePanel>

</div>

function printGrid() {
    var printContent = document.getElementById('<%= table1.ClientID %>');
    var windowUrl = 'about:blank';
    var uniqueName = new Date();
    var windowName = 'Print' + uniqueName.getTime();
    var printWindow = window.open(windowUrl, windowName, 'left=50000,top=50000,width=0,height=0');


    printWindow.document.write(printContent.innerHTML);
    printWindow.document.close();
    printWindow.focus();
    printWindow.print();
    printWindow.close();

}

when click print button(inside panel) modalpop extender automatically hide