ActiveRecord::AssociationNotFoundError: Association named 'author' was not found on Milestone; perhaps you misspelled it?

https://sentry.gitlap.com/gitlab/gitlabcom/issues/115006/

ActiveRecord::AssociationNotFoundError: Association named 'author' was not found on Milestone; perhaps you misspelled it?
  app/models/event_collection.rb:31:in `to_a'
    relation.with_associations.to_a
  app/controllers/projects_controller.rb:307:in `load_events'
    .to_a
  app/controllers/projects_controller.rb:98:in `block (2 levels) in activity'
    load_events
  app/controllers/projects_controller.rb:95:in `activity'
    respond_to do |format|
  lib/gitlab/i18n.rb:47:in `with_locale'
    yield
...
(142 additional frame(s) were not displayed)

ActiveRecord::AssociationNotFoundError: Association named 'author' was not found on Milestone; perhaps you misspelled it?