compatibility issues with engage box

This module gives compatibility problems with a component that I find very necessary, the engage box (www.tassos.gr/joomla-extensions/engagebox).
I have reported the error to them and they reply saying that the problem is in the module, specifically here: JROOT/modules/mod_relateditems_xtd /Helper/ RelatedItemsXtdHelperTag.php: 435

Attached capture.
Can you provide me with a fix? Thank you
 

engage box is a component that allows you to create popup windows with modules and other joomla elements.
What I want is to show the related contents in a pop-up module. I'm doing it with another related module but I wanted to try this one because I like how it works better. And that's when the incompatibility occurs. That is, it works with other modules but not with this one