I'm getting the following warning when trying to print order in Magento 2
Warning: A non-numeric value encountered
It points to line 336 of an extension I'm using. Line 336 contains
$this->lastPageFooterHeight = $fotterHeight + $elementGroupHeight;
As far as I'm aware, there shouldn't be any numerical values on this line. Some research suggests the issue could be with the + in the line. I've also read if could actually be down to the PHP version I'm using (7.3). Can anyone advise?