关闭弹出窗口的Javascript错误

Question

Currently I'm working on ASP.NET project where it's required to have popup. I managed to do the part where it takes input from user but stuck when want to close the pop up.

Here is the error:

0x800a138f - Microsoft JScript runtime error: 'null' is null or not an object

Below is my code:

    function BlockUI(elementID) {
    var prm = Sys.WebForms.PageRequestManager.getInstance();
    prm.add_beginRequest(function () {
        $("#" + elementID).block({ message: '<table align = "center"><tr><td>' +
   '<img src="html/images/loadingAnim.gif"/></td></tr></table>',
            css: {},
            overlayCSS: { backgroundColor: '#000000', opacity: 0.6
            }
        });
    });
    prm.add_endRequest(function () {
        $("#" + elementID).unblock();
    });
   }
   $(document).ready(function () {

    BlockUI("<%=pnlEdit.ClientID %>");
    $.blockUI.defaults.css = {};
  });


   function Hidepopup() {
    $find("popup").hide();
    return false;
  }

Code in aspx.page:-

 <asp:Button ID="btnCancel" runat="server" Text="Cancel" OnClientClick="return     
  Hidepopup()" />

Answer 1

Managed to find the solution for this problem.

Just need to replace this function

      function Hidepopup() {
      $find("popup").hide();
      return false;
      }

with

      function Hidepopup() {
      $find("<%=popup.ClientID %>").hide();
       return false;
      }

Based on my research, the second function is the proper way to write the function for $find,the first function sometimes may return null value.