TypeError: Cannot read property 'getChild' of null - Apps Script - github

Question

I am a newbie and am trying to use a script to send our school website's feeds
to our Google Chat (Google Workspace for Edu).
I found a code here that works like a charm with the testing Url (https://cloudblog.withgoogle.com/products/gcp/rss/),
but returns me an error when I point to our school's website.
TypeError: Cannot read property 'getChild' of null
Here is the code and below the Debug error
// URL of the RSS feed to parse
var RSS_FEED_URL = "https://www.icriccardomassa.edu.it/agid/feed/";
// https://cloudblog.withgoogle.com/products/gcp/rss/"; <- this works!
// Webhook URL of the Hangouts Chat room
var WEBHOOK_URL = "https://chat.googleapis.com/v1/spaces/AAAAueQ0Yzk/messages?key=AI [..]";
// When DEBUG is set to true, the topic is not actually posted to the room
var DEBUG = false;
function fetchNews() {
var lastUpdate = new Date(PropertiesService.getScriptProperties().getProperty("lastUpdate"));
var lastUpdate = new Date(parseFloat(PropertiesService.getScriptProperties().getProperty("lastUpdate")) || 0);
Logger.log("Last update: " + lastUpdate);
Logger.log("Fetching '" + RSS_FEED_URL + "'...");
var xml = UrlFetchApp.fetch(RSS_FEED_URL).getContentText();
var document = XmlService.parse(xml);
// var items = document.getRootElement().getChild('channel').getChildren('item').reverse();
var items = document.getRootElement().getChild('channel').getChildren('item').reverse();
Logger.log(items.length + " entrie(s) found");
var count = 0;
for (var i = 0; i < items.length; i++) {
var pubDate = new Date(items[i].getChild('pubDate').getText());
var og = items[i].getChild('og');
var title = og.getChild("title").getText();
var description = og.getChild("description").getText();
var link = og.getChild("url").getText();
if(DEBUG){
Logger.log("------ " + (i+1) + "/" + items.length + " ------");
Logger.log(pubDate);
Logger.log(title);
Logger.log(link);
// Logger.log(description);
Logger.log("--------------------");
}
if(pubDate.getTime() > lastUpdate.getTime()) {
Logger.log("Posting topic '"+ title +"'...");
if(!DEBUG){
postTopic_(title, description, link);
}
PropertiesService.getScriptProperties().setProperty("lastUpdate", pubDate.getTime());
count++;
}
}
Logger.log("> " + count + " new(s) posted");
}
function postTopic_(title, description, link) {
var text = "*" + title + "*" + "\n";
if (description){
text += description + "\n";
}
text += link;
var options = {
'method' : 'post',
'contentType': 'application/json',
'payload' : JSON.stringify({
"text": text
})
};
UrlFetchApp.fetch(WEBHOOK_URL, options);
}
Thank you in advance for your help!
Debugger errors