Crash in WordPress 5.2 (beta)

Hi,
Trying this on our sandbox – so it is not really critical at this point, but may help you to be ready for 5.2

See error messages below.

An error of type E_ERROR was caused in line 211 of the file /home/andrea47/www/sandbox.peoplesfactory.com/wp-content/plugins/add-on-woocommerce-mailpoet/includes/class-mpwa-frontend-fields.php. Error message: Uncaught Error: Call to undefined function is_checkout() in /home/andrea47/www/sandbox.peoplesfactory.com/wp-content/plugins/add-on-woocommerce-mailpoet/includes/class-mpwa-frontend-fields.php:211 Stack trace: #0 /home/andrea47/www/sandbox.peoplesfactory.com/wp-includes/class-wp-hook.php(286): mpwa_frontend_fields_init_posts_selection(‘ AND wp_posts.p…’) #1 /home/andrea47/www/sandbox.peoplesfactory.com/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters(”, Array) #2 /home/andrea47/www/sandbox.peoplesfactory.com/wp-includes/plugin.php(465): WP_Hook->do_action(Array) #3 /home/andrea47/www/sandbox.peoplesfactory.com/wp-includes/class-wp-query.php(2764): do_action(‘posts_selection’, ‘ AND wp_posts.p…’) #4 /home/andrea47/www/sandbox.peoplesfactory.com/wp-includes/class-wp-query.php(3404): WP_Query->get_posts() #5 /home/andrea47/www/sandbox.peoplesfactory.com/wp-includes/class-wp-query.php(3513): WP_Query->query(Array) #6 /home/andrea47/www/sandbox.peoplesfactory.com/wp-content/plugins/block-lab/php/blo