AttributeError: ‘NoneType’ object has no attribute ‘items’: Can’t upgrade Plone 3 to 4.x

You may have seen an error like the one below when you’ve gone to upgrade your Plone site to something in the Plone 4.x series from Plone 3:

2011-09-14 08:43:32 ERROR plone.app.upgrade Upgrade aborted. Error:
Traceback (most recent call last):
  File "/home/david/.buildout/eggs/Products.CMFPlone-4.1-py2.6.egg/Products/CMFPlone/MigrationTool.py", line 175, in upgrade
    step['step'].doStep(setup)
  File "/home/david/.buildout/eggs/Products.GenericSetup-1.6.3-py2.6.egg/Products/GenericSetup/upgrade.py", line 142, in doStep
    self.handler(tool)
  File "/home/david/.buildout/eggs/plone.app.upgrade-1.1-py2.6 …

in Plone · Wed 14 September 2011

plone.app.blob and Failed Migrations

Another fun-and-games style problem I’ve come across when using plone.app.blob: sometimes migrations won’t work when converting a standard site’s files over to blobs.

That’s a pretty ambiguious description, but essentially, the error you might see will have a semi-normal traceback to start, and then garbage (contents of a file, presumably) - which, depending on the file size might hurt your browser.  The last part of the (normal) traceback reads thus:

File "/home/buildout/instance/eggs/plone.app.linkintegrity-1.0.11-py2.4.egg/ plone/app/linkintegrity/handlers.py", line 158, in referencedObjectRemoved raise LinkIntegrityNotificationException, obj LinkIntegrityNotificationException …

in Plone · Fri 03 July 2009