Fix AttributeError on postcommit when db-create adds non-system CWSource entities In this case, `Repository.add_source` is called from an operation, not from `Repository.init_sources_from_database`, thus the attribute `sources_by_eid` does not exist. Closes #16251078.

addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile