FIX: on topic page, don't try to render post counts for a deleted user

lib/topic_view.rb

@@ -212,6 +212,7 @@ class TopicView
 
   def post_counts_by_user
     @post_counts_by_user ||= Post.where(topic_id: @topic.id)
+                                 .where("user_id IS NOT NULL")
                                  .group(:user_id)
                                  .order("count_all DESC")
                                  .limit(24)