Getting error in AJAX modal popupextender

Hello,

I've an ajax modalpopupextender in an usercontrol, Which displays a list of employees when clicked on search button. I'm not able to see the modalpopupextender when clicked on search button. Getting the below error in debug mode

Microsoft JScript runtime error: Unable to get value of the property 'UI': object is null or undefined 

Sys.Application.add_init(function() {
    $create(Sys.Extended.UI.ModalPopupBehavior, {"CancelControlID":"ModalPopUpAllEHS1_imgclose","OnCancelScript":"hide()","PopupControlID":"ModalPopUpAllEHS1_panelpopup","PopupDragHandleControlID":"ModalPopUpAllEHS1_panelpopup","dynamicServicePath":"/EHSTrack/Personnel/IndustrialHygiene_Add.aspx","id":"ModalPopUpAllEHS1_modalpopupextender1","repositionMode":0}, null, null, $get("ModalPopUpAllEHS1_panelpopup"));
});

Can anyone please help me out in resolving this issue? Not able to find why this issue is coming

Thanks,