NoMethodError: undefined method `sha' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/4103825/?referrer=gitlab_plugin

NoMethodError: undefined method `sha' for nil:NilClass
  app/models/repository.rb:964:in `root_ref_sha'
    @root_ref_sha ||= commit(root_ref).sha
  app/models/repository.rb:955:in `merged_to_root_ref?'
    same_head = branch.target == root_ref_sha
  lib/api/entities/branch.rb:22:in `block in <class:Branch>'
    options[:project].repository.merged_to_root_ref?(repo_branch)
  lib/gitlab/json.rb:130:in `dump'
    Oj.dump(object, opts)
  lib/gitlab/json.rb:130:in `adapter_dump'
    Oj.dump(object, opts)
...
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