Correct identation, default value if note doesn't exist.

app/controllers/projects_controller.rb

@@ -175,9 +175,11 @@ class ProjectsController < ApplicationController
   def participants_in(type, id)
     users = case type
             when "Issue"
-      @project.issues.find_by_iid(id).participants
+              issue = @project.issues.find_by_iid(id)
+              issue ? issue.participants : []
             when "MergeRequest"
-      @project.merge_requests.find_by_iid(id).participants
+              merge_request = @project.merge_requests.find_by_iid(id)
+              merge_request ? merge_request.participants : []
             when "Commit"
               author_ids = Note.for_commit_id(id).pluck(:author_id).uniq
               User.where(id: author_ids)