I'm trying to grab the current tab's icon using Chrome's API and send it via a request:

function sendRequest(update, tab){
    var xhttp = new XMLHttpRequest();
    xhttp.onreadystatechange = function () {
        if (this.readyState == 4 && this.status == 200) {
            console.log(this.responseText);
        }
    };

    icon = "";
    url = "";

    if (tab.status == 'complete')
        icon = tab.favIconUrl;
        url = tab.url;
    
    formD = new FormData();
    formD.append("url", url);
    formD.append("ic_link", icon);
    
    xhttp.open("POST", "Python_server####/send_url");
    xhttp.send(formD);
}

However, the favicon URL is always either blank or undefined. I also tried accessing the link directly with document.getElementById("shortcut icon").href but I'm not sure if that's legal. Any ideas?