Skip if cannot find pipeline, feedback:

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6342/diffs#note_17038807

app/workers/pipeline_notification_worker.rb

@@ -2,7 +2,9 @@ class PipelineNotificationWorker
   include Sidekiq::Worker
   def perform(pipeline_id, recipients = nil)
-    pipeline = Ci::Pipeline.find(pipeline_id)
+    pipeline = Ci::Pipeline.find_by(id: pipeline_id)
+
+    return unless pipeline
     NotificationService.new.pipeline_finished(pipeline, recipients)
   end