New user registration error on website

Hello,

I'm setting up a new user for my WP Network. When they click the registration link I get the following error:
Fatal error: Call to a member function get() on null in /home/tangojam/public_html/wp-content/themes/rt_gantry_wp/header.php on line 14

I'm not even really sure how to go about tracking down this problem.

Thanks!