In the tabbedprefs function, there are some lines that look like this:

legends[0].className = 'mainLegend';
if (legends[0] && legends[0].firstChild.nodeValue) {

sections[seci].text = legends[0].firstChild.nodeValue;

} else {
sections[seci].text = '# ' + seci;
}

The first line up there should be inside an if(legends[0]) block as well to make sure legends[0] exists.

Version: 1.13.x
Severity: enhancement
URL: http://svn.wikimedia.org/viewvc/mediawiki/trunk/phase3/skins/common/prefs.js?view=markup