Addon-SDK : anchor a panel to a widget

main.js

var socialWidget = require("widget").Widget({
    id : "open-clock-btn",
    label : "Clock",
    contentURL : "http://www.mozilla.org/favicon.ico",
    panel : socialPanel,
    onClick : seeingHatenaBookmark
});

function getItemNodeForWidget(widget){
    var item;
    for (var win in require("window-utils").windowIterator()){
        var doc = win.document;
        var bar = doc.getElementById("addon-bar");
        var selector = 'toolbaritem[id$="' + widget.id + '"]';
        item = bar.querySelector(selector);
    }
    return item;
}

var item = getItemNodeForWidget(socialWidget);
socialPanel.show(item);