Midas in XUL througg <editor> does not format text - throw error instead

Hello, all!

I am trying to use Midas through the element of XUL following the
instructions of this article. So far I have the code below:

<window id="main" title="Anunciador Blog Editor" width="300" height="300"
    xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
    xmlns:xhtml="http://www.w3.org/1999/xhtml">
    <script type="application/x-javascript">
    <![CDATA[
    var editor = null;
    function onLoad() {
        editor = document.getElementById('editor');
        editor.contentDocument.designMode = 'on';
    }

    function onBoldButtonCommand() {
        editor.contentDocument.execCommand('bold', false, null);
    }

    window.addEventListener("load", onLoad, false);
    ]]>
    </script>
    <button label="Bold" oncommand="onBoldButtonCommand();" />
    <editor id="editor" type="content-primary" editortype="html"
src="about:blank" flex="1" />
</window>

However, when I click in the "Bold" button with some text selected in
the <editor>, the text is not altered and the JS console presents the
following error:

Error: uncaught exception: [Exception... "Component returned failure
code: 0x80004005
(NS_ERROR_FAILURE) [nsIDOMNSHTMLDocument.execCommand]"  nsresult: "0x80004005
(NS_ERROR_FAILURE)"  location: "JS frame ::
chrome://anunciador/content/main.xul ::
onBoldButtonCommand :: line 14"  data: no]

That does not make sense to me because I have enabled the edit mode with:

editor.contentDocument.designMode = 'on';

Did I forget something?

Thanks in advance!

-- 
Adam Victor Nazareth Brandizzi
http://brandizzi.googlepages.com/