PHP warning in error logs

Hi,
May i know which version you are using and how you get this error.

Version 2.3 free version

I noticed this in Woocommerce system report

Templates

Overrides: /wp-content/plugins/woo-product-gallery-slider/inc/product-image.php

The override file is not the cause of the issue and it’s okay.
Kindly update to the latest version, which is 2.3.2; once you have successfully updated, test the functionality again and verify if the “$html” error still persists.