Question

So I have a chrome extension that checks for an update of programs via an xml document. However i get this error when I open a new tab:

Error in event handler for 'tabs.onActivated': Cannot call method 'getElementsByTagName' of null TypeError: Cannot call method 'getElementsByTagName' of null
at isUpdateAvailable (chrome-extension://bdhjocmpiogdmlfpbpppiffcjbonbocg/background.js:16:17)
at chrome-extension://bdhjocmpiogdmlfpbpppiffcjbonbocg/background.js:84:5
at chrome.Event.dispatch (event_bindings:237:41)
at Object.chromeHidden.Event.dispatchJSON (event_bindings:151:5

Code:

function loadXMLDoc(dname) {
    if (window.XMLHttpRequest) {    
        xhttp=new XMLHttpRequest();
    } else {
        xhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
    xhttp.open("GET",dname,false);
    xhttp.send();
    return xhttp;
}

function isUpdateAvailable(type, build) {
    var buildNumber = localStorage["version" + build + type];
    var xml = loadXMLDoc("http://dl.bukkit.org/api/1.0/downloads/projects/" + build + "/view/latest-" + type + "/");
    var xmlDoc = xml.responseXML;
    var x = xmlDoc.getElementsByTagName("title")[0];
    var y = x.childNodes[0];
    var txt = y.nodeValue;
    if(txt == buildNumber) {
        return true;
    }
    xml.close();
    return false;
}

function notify(type, build) {
    window.webkitNotifications.createNotification('icon.png', getTitle(type, build), getDescription(type, build));
}

function getTitle(type, build) {
    var title;
    var xml = loadXMLDoc("http://dl.bukkit.org/api/1.0/downloads/projects/" + type + "/view/latest-" + build + "/");
    var xmlDoc=xml.responseXML;
    var name=xmlDoc.getElementsByTagName("name")[0];
    var version=xmlDoc.getElementsByTagName("version")[0];
    title+=name;
    title+=" version ";
    title+=version;
    title+=" is out!";
    xml.close();
    return title;
}

function getDescription(type, build) {
    var desc;
    var xml = loadXMLDoc("http://dl.bukkit.org/api/1.0/downloads/projects/" + type + "/view/latest-" + build + "/");
    var xmlDoc=xml.responseXML;
    var name=xmlDoc.getElementsByTagName("name")[0];
    desc+="There is a new update for ";
    desc+=name;
    desc+=". You can download it here: ";
    var downloadLink;
    get_short_url(long_url, function(short_url) {downloadLink=short_url;});
    desc+=downloadLink;
    xml.close();
    return desc;
}

function get_short_url(long_url, func) {
    var login = "kezz101";
    var api_key = "R_d68d87d13b42412a56be9bd9711c4dc4";
    $.getJSON("http://api.bitly.com/v3/shorten?callback=?", 
        { 
            "format": "json",
            "apiKey": api_key,
            "login": login,
            "longUrl": long_url
        },
        function(response) {
            func(response.data.url);
        }
    );
}

chrome.tabs.onActivated.addListener(function() {
    var type = localStorage["type"];
    var build = localStorage["build"];
    if(!type) {
        type = "rb";
    }
    if(!build) {
        build = "craftbukkit";
    }
    if(isUpdateAvailable(type, build)) {
        notify(type, build);
        var xml = loadXMLDoc("http://dl.bukkit.org/api/1.0/downloads/projects/" + type + "/view/latest-" + build + "/");
        var xmlDoc=xml.responseXML;
        localStorage["version" + build + type] = xmlDoc.getElementsByTagName("build_number")[0];
        xml.close();
    }
});

Answer 1

You are using synchronous Ajax, which has been disabled for extensions and apps. You should instead use asynchronous Ajax with a callback passed into loadXMLDoc:

function loadXMLDoc(dname, callback) {
    if (window.XMLHttpRequest) {    
        xhttp=new XMLHttpRequest();
    } else {
        xhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
    xhttp.open("GET",dname);

    xhttp.onload = function() {
        callback(xhttp);
    }

    xhttp.send();
}

function isUpdateAvailable(type, build, callback) {
    var buildNumber = localStorage["version" + build + type];
    loadXMLDoc("http://dl.bukkit.org/api/1.0/downloads/projects/" + build + "/view/latest-" + type + "/", function(xml) {
        var xmlDoc = xml.responseXML;
        var x = xmlDoc.getElementsByTagName("title")[0];
        var y = x.childNodes[0];
        var txt = y.nodeValue;
        if(txt == buildNumber) {
            callback(true);
        } else {
            xml.close();
            callback(false);
        }
    });
}

...

chrome.tabs.onActivated.addListener(function() {
    var type = localStorage["type"];
    var build = localStorage["build"];
    if(!type) {
        type = "rb";
    }
    if(!build) {
        build = "craftbukkit";
    }
    isUpdateAvailable(type, build, function(isAvail) {
        if(isAvail) {
            notify(type, build);
            var xml = loadXMLDoc("http://dl.bukkit.org/api/1.0/downloads/projects/" + type + "/view/latest-" + build + "/", function(xml) {
                var xmlDoc=xml.responseXML;
                localStorage["version" + build + type] = xmlDoc.getElementsByTagName("build_number")[0];
                xml.close();
            });
        }
    });
});