Comment 2 for bug 605513

I can reproduce when I cause the content autocomplete popup to appear (say on google.com), then click somewhere else to dismiss the popup.

onxblpopuphiding in that file is:
http://bonsai.mozilla.org/cvsblame.cgi?file=mozilla/toolkit/content/widgets/autocomplete.xml&rev=1.138#863

and in the content form autocomplete case, this.mInput is a nsFormFillController wrapping the HTMLInputElement. I'm not sure what changed that made this now throw an exception, though - a regression range would be useful.