I’m not sure if it’s a bug or a missconfiguration but since a delete defaut catégories of the demo i get this : http://prntscr.com/h6wcs0
It’s really weird and i would like know how to fix it, or maybe a link to the part of the documentation explaining where i can fix/manage options for this.
Seems that your categories menu has some items with empty titles. We suggest you remove your current categories menu and create a new one with real links in Appearance -> Menus.