Demos with window and without code do not show the previewer when JS is selected

[theCapypara]

The previewer does not show if JavaScript is selected as the language and no code exists. It should instead show the "Show external previewer" button.

Additionally the "Run" button also does nothing in this case. I'm not sure if this is intentional, I think this is normal behaviour for external JS demos already.

Steps to reproduce

1. Select "JavaScript" as the active language on any demo
2. Select a demo without any code but with a Blueprint/XML file where the parent element is a Gtk.Window or subclass. Example: Navigation Split View
3. The demo will open and the internal previewer pane is just empty and the "Run" button does not work.
4. Switching to any other language will resolve this issue. In that case it can even be switched back to JavaScript and Workbench will behave as intended, except for possibly the "Run" button (see above).

[theCapypara]

Actually I was also able to reproduce this issue yesterday with a demo that has no window, so maybe it's not related to having/not having a window.

[sonnyp]

Additionally the "Run" button also does nothing in this case. I'm not sure if this is intentional, I think this is normal behaviour for external JS demos already.

It rerenders and reformats the code only in JS