Erro

Come?ou a dar este erro no plugin, como podemos resolver?

Detalhes do erro
================
Um erro do tipo E_ERROR foi causado na linha 171 do arquivo /home/deliverybestcom/public_html/nostrapizza/wp-content/plugins/new-order-notification-for-woocommerce/new-order-notification-admin.php. Mensagem de erro: Uncaught Error: Call to a member function get_items() on bool in /home/deliverybestcom/public_html/nostrapizza/wp-content/plugins/new-order-notification-for-woocommerce/new-order-notification-admin.php:171
Stack trace:
#0 /home/deliverybestcom/public_html/nostrapizza/wp-includes/class-wp-hook.php(307): new_order_notification_menu('')
#1 /home/deliverybestcom/public_html/nostrapizza/wp-includes/class-wp-hook.php(331): WP_Hook->apply_filters('', Array)
#2 /home/deliverybestcom/public_html/nostrapizza/wp-includes/plugin.php(476): WP_Hook->do_action(Array)
#3 /home/deliverybestcom/public_html/nostrapizza/wp-admin/admin.php(259): do_action('toplevel_page_n...')
#4 {main}
  thrown