Warning — Undefined array key “is_storefront_api”

Hello!

This is Glen from Ecwid by Lightspeed support. One of our customers found this warning by using your plugin:

Warning
Undefined array key "is_storefront_api"

wp-content/plugins/ecwid-shopping-cart/includes/class-ecwid-static-page.php:136
Ecwid_Static_Page::_maybe_fetch_data()
Plugin: ecwid-shopping-cart

Screenshot: https://ecwid.d.pr/i/ycoZU9
Website: https://www.coclico.de/shop/

Can you please help me to understand what does that warning means and how to get rid of it?

Thank you!

The page I need help with: [log in to see the link]