Signup problems. When users try to signup they receive an error

When users try to signup they receive an error:
Fatal error: Uncaught Error: Using $this when not in object context in /var/www/alifbata.dk/public_html/wp-content/plugins/membership/app/addon/mailchimp/class-ms-addon-mailchimp.php:441 Stack trace: #0 /var/www/alifbata.dk/public_html/wp-content/plugins/membership/app/addon/mailchimp/class-ms-addon-mailchimp.php(144): MS_Addon_Mailchimp::is_user_subscribed('test1422222@o2....', '1bab152c25') #1 /var/www/alifbata.dk/public_html/wp-includes/class-wp-hook.php(298): MS_Addon_Mailchimp->subscribe_registered(Object(MS_Model_Event), Object(MS_Model_Member)) #2 /var/www/alifbata.dk/public_html/wp-includes/class-wp-hook.php(323): WP_Hook->apply_filters('', Array) #3 /var/www/alifbata.dk/public_html/wp-includes/plugin.php(453): WP_Hook->do_action(Array) #4 /var/www/alifbata.dk/public_html/wp-content/plugins/membership/app/model/class-ms-model-event.php(546): do_action('ms_model_event_...', Object(MS_Model_Event), Object(MS_Model_Member)) #5 /var/www/alifbata.dk/public_html/wp-content/plugins/membership/app/controller/class-ms-cont in /var/www/alifbata.dk/public_html/wp-content/plugins/membership/app/addon/mailchimp/class-ms-addon-mailchimp.php on line 441

Was trying to fix this with your fix file "class-ms-addon-mailchimp.php" but it does not work.