$(i).attr("src") is undefined

When trying to add a new task the page reloads and Firebug reports the following error in todolist.js (line 213):

$(i).attr("src") is undefined

Here the complete function containing the error:

function getBaseUrl() {
  var url = '';
  var script = $.grep($('head > script'), function(i) {
    return $(i).attr('src').indexOf('todolist.js') > -1
  });
  url = $(script).attr('src');
  url = url.substring(0, url.indexOf('modules/'));
  if (url.indexOf('sites/') > -1)
    url = url.substring(0, url.indexOf('sites/'));
  return url;
}