I can't run my HTML client for Apache Solr

Question

I'm trying to set up Apache Solr and run some experiments. I've set up the Solr search engine on my Ubuntu 18.04, indexed a few files, and it works perfectly given a specific query. The admin page is accessible on the default port on localhost:8983/solr

Now I was trying to setup a web-based client communicating with Solr. I found this simple project located at Google Code here. I extracted it, and follow the simple instructions to make the contents available via localhost http (using python -m http.server). So when I go to http://localhost:8000, I see the index.html client. Apparently the only thing we need to check for integration is js/solr-search.js file to make sure the solrURL is set correctly, which the default is good already:

var solrUrl = 'http://' + document.location.hostname + ':8983/solr/';

But when I try to press the search button, it doesn't show any results and the loading icon keeps showing:

Where is the problem? How can I fix the issue? Would be great if an expert helps me to replicate the same thing.