PHP Fatal error on Nginx error log

Hi, I got some of this error message in the nginx error log. Happened in various product. Any idea how to fix this?

2017/09/20 03:38:23 [error] 28098#28098: *58007 FastCGI sent in stderr: "PHP message: PHP Fatal error:  Uncaught Error: Using $this when not in object context in /var/www/html/example.com/wp-content/plugins/woocommerce-ajax-add-to-cart-for-variable-products/woocommerce-ajax-add-to-cart-variable-products.php:136
Stack trace:
#0 /var/www/html/example.com/wp-includes/class-wp-hook.php(298): woocommerce_add_to_cart_variable_rc_callback('')
#1 /var/www/html/example.com/wp-includes/class-wp-hook.php(323): WP_Hook->apply_filters('', Array)
#2 /var/www/html/example.com/wp-includes/plugin.php(453): WP_Hook->do_action(Array)
#3 /var/www/html/example.com/wp-admin/admin-ajax.php(101): do_action('wp_ajax_nopriv_...')
#4 {main}
  thrown in /var/www/html/example.com/wp-content/plugins/woocommerce-ajax-add-to-cart-for-variable-products/woocommerce-ajax-add-to-cart-variable-products.php on line 136" while reading response header from upstream, client: 172.68.253.214, server: www.example.com, request: "POST /wp-admin/admin-ajax.php HTTP/1.1", upstream: "fastcgi://unix:/run/php/php7.0-fpm.sock:", host: "www.example.com", referrer: "https://www.example.com/shop/some-product/"