export-interface primitive not working in child models

[bainco]

In an LS child model, opened via ls:load-gui-model, the export-interface primitive produces an empty file (with the correct name).

To recreate:

1. Open an LS GUI child model
2. In the command center, enter export-interface "test.png"

The resulting file is created, but empty.

export-view works just fine. Also, perhaps this would be a useful menu item for opened LS models? In regular old NetLogo, you can do File > Export... > Export Interface... but that Menu item is not present when an LS model is the active window.

I'd upload the empty file... but GitHub won't let me.

[bainco]

Coincidentally, when you ls:show <model-id> a child model that was opened via ls:open-headless-model, and then use the command center to try and export the interface with export-interface, it just refuses to do anything at all. No empty file. No error. Nothing. Again, export-view works as expected.