TinyMCE Dissapearing on Widget Update

I’m sure this is simple to fix, but i’m using wp_tiny_mce() function to add TinyMCE onto a <textarea> within a Widget which is created using the normal class structure, i.e

class my_widget extends WP_Widget {
    widget(){
    }

    update() {
    }

    form(){
        wp_tiny_mce( true , array("editor_selector" => $this->get_field_id('html_content')));
    }
}

Everything works great but when I hit Save the TinyMCE editor dissapears and you just see the normal textarea again with it’s old content?

The Save button fires an AJAX Post and so I assume this is just javascript not being loaded. Firebug shows no JS errors though?

Any ideas what is happening here?