Javascript Object Required error when scrolling page.

Question

I have a page that uses the ModalPopupExtender. The popup can do some functions on it, and is kept open using ModalPopupExtender.Show() server-side.

Whenever I do something other than closing the popup, I start getting javascript "object required" errors at line 2, but only when I try to scroll up and down the page.

The popup is opened when you select a row in a GridView. There is also some custom Javascript to display a Loading indicator over the clicked row.

I've already tried deleting the local files.

In general, what are some other things I can do to fix this problem?