Fatal Error when clicking Blog Posts (or pre-existing pages)

Hi,

I have installed the Fixer theme...Things are coming along quite nicely!
However I have heaps of blog posts that existed before I activated the theme and they are returning this error:

Fatal error: Class 'WP_Screen' not found in /home/wheeliek/public_html/wp-content/themes/upfront/elements/upfront-this-post/lib/this_post.php on line 539

Please advise.

Thanks,
Brett