Preview window doesn't close sometimes

This shows up with the current state of #744 and #742. When you run the demo, the preview window opens, but then you can't close it, even if you close all other Workbench windows.

When running such previews, there's a warning message in the terminal:

(process:196): Gtk-WARNING **: 18:11:28.728: A window is shown after it has been destroyed. This will leave the window in an inconsistent state.

[theCapypara]

Looking at the affected demos, this is most likely an issue with demos that have windows as top-level components (meaning the previewer doesn't explicitly have to create it).