I have the same error. This error is outputted several times in the result of admin-ajax.php and breaks the media manager when used in a advanced custom fields image field (with flexible content plugin).
http://pastebin.com/Vxf8RmVq
WordPress Version 3.9.1
Custom Post Type Maker Version 0.1.3
Advanced Custom Fields Version 4.3.8
Advanced Custom Fields: Flexible Content Field Version 1.1.1
