Which fails validation:
ValidationError: None is not of type u'string'
Version: unspecified
Severity: major
Which fails validation:
ValidationError: None is not of type u'string'
Version: unspecified
Severity: major
Related URL: https://gerrit.wikimedia.org/r/69966 (Gerrit Change I1626af38640c921e616be05f93d42ba795eaaf01)
I researched this more carefully and determined that this error last occurred in April, with revision 5303086 of the schema (https://meta.wikimedia.org/wiki/Schema:PageContentSaveComplete?oldid=5303086).
It could be edifying to go back and figure out the exact source of the bug, but I don't expect to have the time for it.