Cursor moving problem

I switch the window after loading sitpal on previous window.
When i move mouse on new window it givs following error on following function:

Error: (Microsoft JScript runtime error: Unable to get value of the property 'offsetLeft': object is null or undefined)

function AnchorPosition_getPageOffsetLeft (el) {
var ol=el.offsetLeft;
while ((el=el.offsetParent) != null) { ol += el.offsetLeft; }
return ol;
}