PHP Error when activating Hustle

Hi Support,
We seem to be getting a PHP error when we try to activate any of the pop ups within Hustle.

The PHP error we are getting is:

PHP Fatal error: Uncaught Error: Call to a member function get_error_message() on integer in /dom19263/wp-content/plugins/hustle/inc/hustle-custom-content-admin-ajax.php:111
Stack trace:
#0 /wordpress-4.7.3/wp-includes/class-wp-hook.php(298): Hustle_Custom_Content_Admin_Ajax->toggle_test_activity('')
#1 /wordpress-4.7.3/wp-includes/class-wp-hook.php(323): WP_Hook->apply_filters('', Array)
#2 /wordpress-4.7.3/wp-includes/plugin.php(453): WP_Hook->do_action(Array)
#3 /wordpress-4.7.3/wp-admin/admin-ajax.php(91): do_action('wp_ajax_hustle_...')
#4 {main}
thrown in /dom19263/wp-content/plugins/hustle/inc/hustle-custom-content-admin-ajax.php on line 111
Any ideas why this is happening or any way around it?

Many Thanks

Chris