Interface fix

[pngwn]

Some small tweaks to fix a few errors i came across with interfaces.

[abidlabs]

Thanks @pngwn! What errors does this PR fix? That way, I can test them and link potentially related issues

[pngwn]

It was runtime errors when running the app. A few of those cannnot get length of undefined issues. It was just related to components that don't have children. I also fixed the HighlightedText: one was a regression (named colours working as part of the color_map #724 ), I think the other was #887. The carousel needs some other work, including on the backend.

The default_value changes were a pre-emptive fix because changing the value prop to default_value on the frontend will break event triggers.

[pngwn]

In terms of testing, the kitchen_sink should work (apart from the carousel although it shouldn't error).

[abidlabs]

Tested and LGTM!