I think this is a bug in Plone 'somewhere', since i had the same error in 5.0.4. .
webcoutier dropdownmenu was also complaining about missing 'item_icon"
I just removed item_icon from the template and did not think about it anymore
PS: Be sure to check that you dont have any customized templates ( in /portal_skins/custom or in /portal_view_customization
..... or in your theme for that matter.