DEV: ensure compatibilty with stable

plugin.rb

@@ -69,6 +69,7 @@ after_initialize do
       end
     end
 
+    if TopicQuery.respond_to?(:results_filter_callbacks)
       TopicQuery.results_filter_callbacks << ->(_type, result, user, options) {
         result = result.includes(:topic_vote_count)
 
@@ -89,6 +90,7 @@ after_initialize do
 
         result
       }
+    end
 
     add_to_serializer(:category, :custom_fields) do
       object.custom_fields.merge(enable_topic_voting: DiscourseVoting::CategorySetting.find_by(category_id: object.id).present?)