NoMethodError: undefined method `authorized_groups' for nil:NilClass

https://sentry.gitlab.net/gitlab/gitlabcom/issues/797540/

NoMethodError: undefined method `authorized_groups' for nil:NilClass
  users/participable_service.rb:32:in `groups'
    .of_groups(current_user.authorized_groups)
  projects/participants_service.rb:10:in `execute'
    participants = noteable_owner + participants_in_noteable + all_members + groups + project_members
  gitlab/metrics/instrumentation.rb:161:in `block in execute'
    .measure { super }
  gitlab/metrics/method_call.rb:36:in `measure'
    retval = yield
  gitlab/metrics/instrumentation.rb:161:in `execute'
    .measure { super }
...
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NoMethodError: undefined method `authorized_groups' for nil:NilClass