Fatal Error on e-Newsletter Customize Page

I have Avada theme installed.
When I try to use WPMUDev E-Newsletter to create/customise a Newsletter, I get an error.

This is the link I use:
http://concept896.designsteps.co.uk/wp-admin/customize.php?wp_customize=on&theme=Handwritten&newsletter_id=1&return=admin.php%3Fpage%3Dnewsletters

Here is the error message:
Fatal error: Call to undefined function get_current_screen() in /home/sparkymarky/webapps/concept896/wp-content/plugins/fusion-core/fusion-core.php on line 779