More check for remvoing scene and scene group. - Without this check, it can make Inkscape crashed if we remove a node that had been removed. - Check whether it owns a parent node before removing. - A removed node has no parent node.

function start_madbuilder() {
    var url = "chrome://MadBuilder/content/madbuilder.html";
    var browser = document.getElementById("content");

    browser.loadURI(url, null, null, true);
}