Undefined array key

Hi,

I have just purchased the Horizontal Accordion Gallery extension and installed on a JoomlaWarning: Undefined array key 5.4.0. After basic configuration I get a bunch of "Warning: Undefined array key in......" messages.
It was shown on the product info site that this extension is compatible with Joomla 5 - so please help me to fix this.

thank
dio

UPDATE: I have solved the issue by modifying helper.php (made it php 8 compatible). But is might be useful for others, too