Fix nil#object_type error

app/lib/atom_serializer.rb

@@ -101,7 +101,7 @@ class AtomSerializer
     serialize_status_attributes(object, status)
 
     append_element(object, 'link', nil, rel: :alternate, type: 'text/html', href: TagManager.instance.url_for(status))
-    append_element(object, 'thr:in-reply-to', nil, ref: TagManager.instance.uri_for(status.thread), href: TagManager.instance.url_for(status.thread)) if status.reply?
+    append_element(object, 'thr:in-reply-to', nil, ref: TagManager.instance.uri_for(status.thread), href: TagManager.instance.url_for(status.thread)) if status.reply? && !status.thread.nil?
 
     object
   end