FIX: Don't preload unless the plugin is enabled

plugin.rb

@@ -41,6 +41,7 @@ after_initialize do
   end
 
   TopicList.on_preload do |topics, topic_list|
+    if SiteSetting.assign_enabled?
       is_staff = topic_list.current_user && topic_list.current_user.staff?
       allowed_access = SiteSetting.assigns_public || is_staff
 
@@ -63,6 +64,7 @@ after_initialize do
         end
       end
     end
+  end
 
   require_dependency 'topic_query'
   TopicQuery.add_custom_filter(:assigned) do |results, topic_query|