Custom Shortcode and Button not Working after 3.9 update

Try like this:

<script type="text/javascript">
    tinymce.init({
    selector: "textarea",
    toolbar: "mybutton",
    setup: function(editor) {
        editor.addButton('mybutton', {
            type: 'splitbutton',
            text: 'My button',
            icon: false,
            onclick: function() {
                editor.insertContent('Main button');
            },
            menu: [
                {text: 'Menu item 1', onclick: function() {
                                  tinyMCE.activeEditor.execCommand("myPopup", false, {
                                      title: 'Divider',
                                      identifier: 'divider'
                              })}},
                {text: 'Menu item 2', onclick: function() {editor.insertContent('Menu item 2');}}
            ]
        });
    }
    });