I'm building a website using Django and I want to add this image gallery to one of my pages, but I'm having issues integrating the code.

The HTML, CSS, and JS code I want to add is here: https://gist.github.com/CodeMyUI/9769b75c7a81d24138ae74bdbadc39e4

I've added the JS (inside a <script> element) and HTML to my django template, and added the CSS to my project's style.css file, but when I try to render, I'm getting a console error:

Uncaught TypeError: Cannot read properties of null (reading 'classList')

I also don't see anywhere in the code snippets that points to the images that I want to use for the gallery. Help on what I'm missing to successfully integrate this cool image gallery would be appreciated.