Hi, I got the same message when I clicked on the XML Sitemap link on a brand new install. Clicking on the link is supposed to display the XML sitemap... but I hadn't yet clicked on Save so the sitemap hadn't been created. The Undefined property message was displayed on the subsequent 404 page. Given that the default is for the XML sitemap button to be checked perhaps there should be some logic to create the XML sitemap before the page is first displayed... so long as the XML sitemap option is not yet set.
The XML sitemap problem is a separate problem. I still needed to deselect the checkbox to remove the message.