I am too having the same issue: Fatal error: Call to a member function get_post_types() on a non-object in /home/itgdesig/public_html/intechgrity.com/wp-content/plugins/yet-another-related-posts-plugin/class-admin.php on line 237
I think, it is happening mainly to the drafted posts. I am not sure though. Coz for most of the new posts I write, it works fine. But when I edit a draft which was saved long ago (some of them are even 6-10 months old and might have been created before installing YARPP, but I am not sure) this thing happens.