fix for post archive presenter

app/models/post_archive.rb

@@ -98,7 +98,7 @@ class PostArchive < ActiveRecord::Base
   def previous
     # HACK: if all the post versions for this post have already been preloaded,
     # we can use that to avoid a SQL query.
-    if association(:post).loaded? && post.association(:versions).loaded?
+    if association(:post).loaded? && post && post.association(:versions).loaded?
       post.versions.sort_by(&:version).reverse.find { |v| v.version < version }
     else
       PostArchive.where("post_id = ? and version < ?", post_id, version).order("version desc").first