[Plugin: Related Posts Thumbnails] PHP fatal error on all new blog posts

This plugin has been working perfectly for me, however when I went to create a new blog post today and went to preview it (before any images were inserted), I got this error:

`
Fatal error: Call to a member function cached_singlepic_file() on a non-object in /websites/123reg/LinuxPackage21/tr/ea/nh/treanhouse.com/public_html/wp-content/plugins/related-posts-thumbnails/related-posts-thumbnails.php on line 259`

It seems this now appears for any new blogpost I try to create, even with just a subject and some body text. The posts will display fine in the main blogroll, but when clicking into the single post (where related thumbnails are used), I get this error.

I use Nggallery to store and manage all my images, and looking at the line in the related-posts-thumbnails.php file, it does have something to do with Nggallery. I emptied Nggallery’s thumbnail cache but hasn’t made any difference. Existing blog posts made before today display just fine. I’m fairly sure I didn’t update Nggallery or Related Posts Thumbnails recently. Using version 1.3.1 of the plugin.

Any ideas on how to fix this?

Thanks.

http://wordpress.org/extend/plugins/related-posts-thumbnails/