PHP Warning: Undefined array key “_zaddon_additional”

Version info:
PAOfWC: 2.1.38
WC 6.3.1
WP 5.9.2
PHP 8.0.17

Another unrelated, but this time critical error I’ve found in the log:

CRITICAL Uncaught Error: Call to a member function get_category_ids() on bool in /srv/wp-content/plugins/product-add-ons-woocommerce/includes/Model/Group.php:344
Stack trace:
#0 /srv/wp-content/plugins/product-add-ons-woocommerce/includes/Frontend/Product.php(521): ZAddons\Model\Group::getByProduct()
#1 /srv/wp-includes/class-wp-hook.php(309): ZAddons\Frontend\Product->add_to_cart_validation()
#2 /srv/wp-includes/plugin.php(189): WP_Hook->apply_filters()
#3 /srv/wp-content/plugins/woocommerce/includes/class-wc-form-handler.php(904): apply_filters()
#4 /srv/wp-content/plugins/woocommerce/includes/class-wc-form-handler.php(790): WC_Form_Handler::add_to_cart_handler_variable()
#5 /srv/wp-includes/class-wp-hook.php(307): WC_Form_Handler::add_to_cart_action()
#6 /srv/wp-includes/class-wp-hook.php(331): WP_Hook->apply_filters()
#7 /srv/wp-includes/plugin.php(474): WP_Hook->do_action()
#8 /srv/wp-settings.php(609): do_action()
#9 /srv/wp-config.php(128): require_once('...')
#10 /srv/wp-load.php(50): require_once('...')
#11 /srv/wp-blog-header.php(13): require_once('...')
#12 /srv/index.php(17): require('...')
#13 {main}
  thrown in /srv/wp-content/plugins/product-add-ons-woocommerce/includes/Model/Group.php in line 344

Any response?

I checked a bit more, and it can actually be two warnings about faulty accesses:

PHP Warning: Undefined array key “_zaddon_additional” in /srv/wp-content/plugins/product-add-ons-woocommerce/includes/Frontend/Product.php on line 357
PHP Warning: Undefined array key “_zaddon_additional” in /srv/wp-content/plugins/product-add-ons-woocommerce/includes/Frontend/Product.php on line 272

That’s $cart_item['_zaddon_additional'] = $values['_zaddon_additional'] and $cart_item['_zaddon_additional'] > 0 respectively.

Both of them seem (at least) related to certain products being added to the cart, and then each time the cart site is opened.

Another:

PHP Warning: Undefined array key “_zaddon_additional” in /srv/wp-content/plugins/product-add-ons-woocommerce/includes/Frontend/Product.php on line 227