RepositoryUpdateMirrorWorker::UpdateMirrorError: NoMethodError: undefined method `sha' for nil:Ni...

https://sentry.gitlap.com/gitlab/gitlabcom/issues/10167/

NoMethodError: undefined method `sha' for nil:NilClass
  app/services/projects/update_mirror_service.rb:76:in `block in update_tags'
    tag_target = tag.target.sha
  app/services/projects/update_mirror_service.rb:74:in `each'
    tags.each do |tag|
  app/services/projects/update_mirror_service.rb:74:in `update_tags'
    tags.each do |tag|
  lib/gitlab/metrics/instrumentation.rb:155:in `block in update_tags'
    trans.method_call_for(#{label.to_sym.inspect}).measure { super }
  lib/gitlab/metrics/method_call.rb:23:in `measure'
    retval = yield
...
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RepositoryUpdateMirrorWorker::UpdateMirrorError: NoMethodError: undefined method `sha' for nil:NilClass
  app/workers/repository_update_mirror_worker.rb:26:in `rescue in perform'
    raise UpdateMirrorError, "#{ex.class}: #{Gitlab::UrlSanitizer.sanitize(ex.message)}"
  app/workers/repository_update_mirror_worker.rb:12:in `perform'
    begin
  lib/gitlab/sidekiq_middleware/request_store_middleware.rb:6:in `call'
    yield
  lib/gitlab/sidekiq_middleware/memory_killer.rb:17:in `call'
    yield
  lib/gitlab/sidekiq_middleware/arguments_logger.rb:6:in `call'
    yield
...
(26 additional frame(s) were not displayed)