Error trying to delete a reference with an empty source during upgrade to Plone 5

[fulv]

I'm running an upgrade from 4.0.10 to 5.0, via an intermediate 4.3.7 step. While on 4.3.7 I migrate all content to plone.app.contenttypes before starting the 5.0 upgrade.

On 5.0, the upgrade fails with the following traceback:

2015-12-07 22:40:56 ERROR plone.app.upgrade Upgrade aborted. Error:
Traceback (most recent call last):
  File "/Users/fulvio/Downloads/Plone-5.0/buildout-cache/eggs/Products.CMFPlone-5.0-py2.7.egg/Products/CMFPlone/MigrationTool.py", line 257, in upgrade
    step['step'].doStep(setup)
  File "/Users/fulvio/Downloads/Plone-5.0/buildout-cache/eggs/Products.GenericSetup-1.8.0-py2.7.egg/Products/GenericSetup/upgrade.py", line 166, in doStep
    self.handler(tool)
  File "/Users/fulvio/Downloads/Plone-5.0/buildout-cache/eggs/plone.app.upgrade-1.3.18-py2.7.egg/plone/app/upgrade/v50/betas.py", line 353, in to50rc1
    migrate_linkintegrity_relations(portal)
  File "/Users/fulvio/Downloads/Plone-5.0/buildout-cache/eggs/plone.app.linkintegrity-3.0.2-py2.7.egg/plone/app/linkintegrity/upgrades.py", line 21, in migrate_linkintegrity_relations
    source_obj, target_obj, relationship=referencedRelationship)
  File "/Users/fulvio/Downloads/Plone-5.0/buildout-cache/eggs/Products.Archetypes-1.10.10-py2.7.egg/Products/Archetypes/ReferenceEngine.py", line 343, in deleteReference
    sID, sobj = self._uidFor(source)
  File "/Users/fulvio/Downloads/Plone-5.0/buildout-cache/eggs/Products.Archetypes-1.10.10-py2.7.egg/Products/Archetypes/ReferenceEngine.py", line 533, in _uidFor
    raise ReferenceException, "%r not referenceable" % uobject
ReferenceException: None not referenceable

In the plone.app.linkintegrity.upgrades.migrate_linkintegrity_relations call, brain has two non-empty values for sourceUID and targetUID, but uuidToObject(brain.sourceUID) produces None, while uuidToObject(brain.targetUID) is a jpg image object.
After that, reference_catalog.deleteReference is called with a None source_obj, and from there things go downhill.

I don't know why brain.sourceUID does not resolve to a valid object, and I don't know what should be done about it, but it seems that somewhere along the line there should be a safeguard against this case.

Also, if anyone has any suggestions on how to get around this so I can finish my upgrade, I would be much obliged.

[fulv]

Ok, I got around it by clearing the reference_catalog in Plone 4 before doing the upgrade.

[gforcada]

@fulv something to be added on the upgrade documentation?

[Martronic-SA]

I think clearing catalog is a workaround but not the solutions as ther are certainly references to keep.
You should just avoid migrating the ones that points to None with a try except / continue and migrating the good ones.