Tampermonkey userscript prevents pages from loading

I am currently making a userscript to interpret the APL programming language in a Stackexchange chat window. This is the code I have come up with:

// ==UserScript==
// @name     APL chat
// @version  2
// @grant    GM_xmlhttpRequest
// @grant    GM_listValues
// @match    https://chat.stackexchange.com/*
// @run-at   document-start
// @require  https://cdn.jsdelivr.net/npm/apl@0.1.15/lib/apl.min.js
// ==/UserScript==

// thanks to @cvzi for making it work correctly!

var MutationObserver = window.MutationObserver || window.WebKitMutationObserver;

var observer = new MutationObserver(function(mutations, observer) {
    // fired when a mutation occurs
    console.log(mutations, observer);
    let codes = document.getElementsByTagName("code");
        for (let elem of codes) {
            if(elem.innerText && !('interpreted' in elem.dataset) && elem.innerText[0] == '⋄') {
                elem.dataset.interpreted = true;  // see https://developer.mozilla.org/en-US/docs/Learn/HTML/Howto/Use_data_attributes#JavaScript_access
                let result = ''
                let color = 'red'
                // Catch apl error and show it in orange color
                try {
                    result = apl(elem.innerText).toString()
                } catch(e) {
                    result = e.toString()
                    color = 'orange'
                }
                let tmp = document.createElement("div");
                tmp.innerHTML = "<pre style=\"color:"+color+"\">"+result.replace("\n","<br>")+"</pre>";
                let parent = elem.parentElement;
                parent.appendChild(tmp.firstChild);
                //console.log(result);
            }
        }
});

window.addEventListener('DOMContentLoaded', (event) => {
    alert = function() {}; // Prevents ⎕← and ⍞← from trggering alerts
    window.alert = function(){};
      observer.observe(document, {
      subtree: true,
    ChildList:true,
      attributes: true
    });

});

When a code block is written to the body with a ⋄ character in it, it should be executed and displayed in red, underneath the code.

The apl() function takes a single string, and interprets in in the APL language, and it is imported from the @require statement, using ngn's javascript APL interpreter. After ngn's confirmation, I can say that it does not interfere with any globals.

It works perfectly on firefox(Greasemonkey), but on Chrome's Tampermonkey, any chat.stackexchange.com page I visit never stops loading. It gets stuck at this point:

And these are the console errors:

Uncaught TypeError: o.substr is not a function
    at HTMLLIElement.<anonymous> (master-chat-with-millinery.js?v=93a5b9100f35:1)
    at Function.each (jquery.min.js:2)
    at n.fn.init.each (jquery.min.js:2)
    at o (master-chat-with-millinery.js?v=93a5b9100f35:1)
    at Sidebar (master-chat-with-millinery.js?v=93a5b9100f35:1)
    at Vt (master-chat-with-millinery.js?v=93a5b9100f35:3)
    at StartChat (master-chat-with-millinery.js?v=93a5b9100f35:3)
    at HTMLDocument.<anonymous> (the-nineteenth-byte:182)
    at i (jquery.min.js:2)
    at Object.fireWith [as resolveWith] (jquery.min.js:2)
master-chat-with-millinery.js?v=93a5b9100f35:7 Uncaught TypeError: Cannot read property 'resolve' of undefined
    at Object.<anonymous> (master-chat-with-millinery.js?v=93a5b9100f35:7)
    at i (jquery.min.js:2)
    at Object.fireWith [as resolveWith] (jquery.min.js:2)
    at y (jquery.min.js:4)
    at XMLHttpRequest.c (jquery.min.js:4)
DevTools failed to load SourceMap: Could not load content for chrome-extension://dhdgffkkebhmkfjojejmpbldmpobfkfo/sm/66503b0e4799e2375dd4e11269326fc0fcb9a65474ff19f45c7f956fd381cea8.map: HTTP error: status code 404, net::ERR_UNKNOWN_URL_SCHEME

I have my suspicions on the MutationObserver and the @grant, but I'm not sure what exactly is causing this issue. It could be something else.

What is the correct fix for this problem?

from Tampermonkey userscript prevents pages from loading