About Project Item's ID

There's default method app.project.itemByID(itemID) that get's project item by ID. However, it will raise error if such itemID is not valid. To overcome this, use it in try/catch block.

var itemID = 1;
var item = getItemByID(itemID);
alert(item);
function getItemByID(itemID) {
    try {
        return app.project.itemByID(itemID);
    } catch(e) {
        return null;
    }
}

Hope that helps.