getting error in java script

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Javascript

Hi i am getting below error in javascript:

Microsoft JScript runtime error: 'scrollLeft' is null or not an object

Error is getting only when I have added ClientIDMode="AutoID" this property to child page directive

this is the script:

window.moveTo(0, 0);
       window.resizeTo(screen.availWidth, screen.availHeight);
       var xPos, yPos;
       var prm = Sys.WebForms.PageRequestManager.getInstance();
       prm.add_beginRequest(BeginRequestHandler);
       prm.add_endRequest(EndRequestHandler);
       function BeginRequestHandler(sender, args) {
           xPos = $get('contentPanel').scrollLeft;
           yPos = $get('contentPanel').scrollTop;
       }
       function EndRequestHandler(sender, args) {
           $get('contentPanel').scrollLeft = xPos;
           $get('contentPanel').scrollTop = yPos;
       }

Please guide me resolve this issue.