Loading merge request with removed source branch results in exception

Summary

Loading a merge request with removed source branch results in exception NoMethodError: undefined method 'raw' for nil:NilClass

Steps to reproduce

1. Navigate to branches
2. Delete branch associated with existing merge request
3. Load merge request

What is the current bug behavior?

NoMethodError: undefined method 'raw' for nil:NilClass
  from lib/gitlab/conflict/file_collection.rb:10:in `initialize'
  from app/models/merge_request.rb:973:in `new'
  from app/models/merge_request.rb:973:in `conflicts'
  from app/models/merge_request.rb:993:in `conflicts_can_be_resolved_in_ui?'
  from lib/gitlab/metrics/instrumentation.rb:156:in `block in conflicts_can_be_resolved_in_ui?'
  from lib/gitlab/metrics/method_call.rb:23:in `measure'
  from lib/gitlab/metrics/instrumentation.rb:156:in `conflicts_can_be_resolved_in_ui?'
  from delegate.rb:83:in `method_missing'

What is the expected correct behavior?

Load the merge request and display message saying the source branch is removed.

Relevant logs and/or screenshots

• https://sentry.gitlap.com/gitlab/gitlabcom/issues/30544/

@smcgivern

mentioned in issue #32437 (closed)

changed the description

changed milestone to %9.2

@adamniedzielski Could you have a look at this one maybe?

mentioned in issue #32544 (closed)

Fixed in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11501

closed