I helped someone else just the other day who was getting this error after upgrading from 2.3.5.8 to 2.3.6.5.
It is something strange because the upgrade scripts have sql statements to make sure the new fields are not null, but for some reason it seems like it did not run those parts of the upgrade scripts. I fixed it for him by running the same update statements manually from sql management query tool. If you run this it should solve the problem.
UPDATE mp_Modules
SET HeadElement = 'h2'
GO
UPDATE mp_Pages
SET ExpandOnSiteMap = 1
GO
UPDATE mp_Modules
SET IsGlobal = 0
GO
UPDATE mp_Pages
SET IncludeInChildSiteMap = IncludeInSiteMap
GO
UPDATE mp_Pages
SET PubTeamId = '00000000-0000-0000-0000-000000000000'
GO
Hope that helps,
Joe