Here's what I have:
1) Plone 5.0.6.
2) I get this error constantly in my log:
2016-11-11T15:00:38 ERROR Zope.SiteErrorLog 1478876438.090.780612564538
Traceback (innermost last):
Module ZPublisher.Publish, line 138, in publish
Module ZPublisher.mapply, line 77, in mapply
Module ZPublisher.Publish, line 48, in call_object
Module Products.CMFPlone.browser.robots, line 24, in call
Module plone.memoize.view, line 67, in memogetter
Module plone.app.layout.globals.portal, line 48, in portal_url
AttributeError: 'NoneType' object has no attribute 'absolute_url'
3) I basically understand what robots.txt does and how to configure in Plone
4) Specific to Plone, I found this:
(look at last answer). Note: I know this is old; however, didn't find anything newer.
5) I looked here: https://github.com/plone/plone.app.layout/blob/master/plone/app/layout/globals/interfaces.py
This was the closest I could find (but missing "portal" part of the search). couldn't find anything.
6) this error does not appear to cause problems. However, is this a bug? If this is a bug, where should I report it?