Same issue here with version 1.1.3:
[20-Apr-2019 11:21:22 UTC] PHP Fatal error: Uncaught Error: Call to undefined function is_checkout() in /home/xxx/public_html/wp-content/plugins/add-on-woocommerce-mailpoet/includes/class-mpwa-frontend-fields.php:211
Stack trace:
#0 /home/xxx/public_html/wp-includes/class-wp-hook.php(286): mpwa_frontend_fields_init_posts_selection(' AND wp_posts....')
#1 /home/xxx/public_html/wp-includes/class-wp-hook.php(310): WP_Hook->apply_filters('', Array)
#2 /home/xxx/public_html/wp-includes/plugin.php(465): WP_Hook->do_action(Array)
#3 /home/xxx/public_html/wp-includes/class-wp-query.php(2747): do_action('posts_selection', ' AND wp_posts....')
#4 /home/xxx/public_html/wp-includes/class-wp-query.php(3387): WP_Query->get_posts()
#5 /home/xxx/public_html/wp-includes/post.php(1961): WP_Query->query(Array)
#6 /home/xxx/public_html/wp-content/plugins/schema-and-structured-data-for-wp/view/post_specific.php(68): get_posts(Array)
#7 /home/xxx/public_html/wp-cont in /home/xxx/public_html/wp-content/plugins/add-on-woocommerce-mailpoet/includes/class-mpwa-frontend-fields.php on line 211
Kind regards.