NoMethodError: ProcessCommitWorker undefined method group

Sentry error: https://new-sentry.gitlab.net/organizations/gitlab/issues/1770607

NoMethodError: undefined method `group` for #<WorkItem id: xxx#xxx> (NoMethodError)
  from active_model/attribute_methods.rb:511:in `method_missing'
  from active_record/gitlab_patches/attribute_methods.rb:32:in `method_missing'
  from app/services/system_notes/issuables_service.rb:254:in `cross_reference'
  from app/services/system_note_service.rb:262:in `cross_reference'
  from app/models/concerns/mentionable.rb:150:in `block in create_cross_references!'
  from app/models/concerns/mentionable.rb:149:in `each'
  from app/models/concerns/mentionable.rb:149:in `create_cross_references!'
  from app/workers/process_commit_worker.rb:55:in `process_commit_message'
  from app/workers/process_commit_worker.rb:43:in `perform'

Problem

I spotted failures coming from ProcessCommitWorker. They caused by group level WorkItem system notes that don't implement group method.

Proposal

Handle the case when mentionable object doesn't implement group.