HtmlEditorField duplicates itself after being reordered in Widgets currently used

[torleif]

Steps to reproduce:

1. create a widget that returns a HtmlEditorField in getCMSFields:

    public function getCMSFields() {
        return FieldList::create(
                        TextField::create('Title'), HtmlEditorField::create('Content')
        );
    }

2. Create the widget on a page with widgets
3. reorder the widget in the page: