ActiveRecord::RecordInvalid: Failed to set ci_pipelines: Validation failed: Merge request can't be blank

https://sentry.gitlab.net/gitlab/gitlabcom/issues/911525/

ActiveRecord::RecordInvalid: Validation failed: Merge request can't be blank
  active_record/validations.rb:80:in `raise_validation_error'
    raise(RecordInvalid.new(self))
  active_record/validations.rb:52:in `save!'
    perform_validations(options) ? super : raise_validation_error
  active_record/transactions.rb:315:in `block in save!'
    with_transaction_returning_status { super }
  active_record/transactions.rb:387:in `block in with_transaction_returning_status'
    status = yield
  active_record/connection_adapters/abstract/database_statements.rb:265:in `transaction'
    yield
...
(114 additional frame(s) were not displayed)

ActiveRecord::RecordInvalid: Failed to set ci_pipelines: Validation failed: Merge request can't be blank
  project.rb:1910:in `rescue in append_or_update_attribute'
    raise e, "Failed to set #{name}: #{e.message}"
  project.rb:1892:in `append_or_update_attribute'
    def append_or_update_attribute(name, value)
  gitlab/metrics/instrumentation.rb:161:in `block in append_or_update_attribute'
    .measure { super }
  gitlab/metrics/method_call.rb:36:in `measure'
    retval = yield
  gitlab/metrics/instrumentation.rb:161:in `append_or_update_attribute'
    .measure { super }
...
(84 additional frame(s) were not displayed)

ActiveRecord::RecordInvalid: Failed to set ci_pipelines: Validation failed: Merge request can't be blank

Possible workaround

change "source": "merge_request_event" to "source": "push" in project.json until !17844 (merged) is made available from #31414 (comment 223841197)