E_ERROR

Error details
===========
An error of type E_ERROR was caused on line number 1593 of the file /public_html/wp-content/plugins/newsletter-optin-box/includes/functions.php. Foutmelding: Uncaught Error: Cannot use a scalar value as an array in /public_html/wp-content/plugins/newsletter-optin-box/includes/functions.php:1593

Stack trace:

#0 
/public_html/wp-content/plugins/newsletter-optin-box/templates/settings.php(47): noptin_kses_post_vue()

#1 
/public_html/wp-content/plugins/newsletter-optin-box/includes/functions.php(370): include('...')

#2 
/public_html/wp-content/plugins/newsletter-optin-box/includes/admin/class-noptin-settings.php(47): get_noptin_template()

#3 
/public_html/wp-includes/class-wp-hook.php(308): Noptin_Settings::output()

#4 
/public_html/wp-includes/class-wp-hook.php(332): WP_Hook->apply_filters()

#5 
/public_html/wp-includes/plugin.php(517): WP_Hook->do_action()

#6 
/public_html/wp-admin/admin.php(259): do_action()

#7 {main}

  thrown

So I got this error this morning.
When I want to change the status of a WooCommerce order to complete, I get the critical error page. The order does get the new status, but The error page is not a good thing.

How to solve this?