Fixes #18944: Clearing widget type field no longer causes 500 error

[jnovinger]

Fixes: #18944

I don't love this fix since the feedback to the user is pretty bad--it just resets the form to defaults without explaining what happened. I didn't find an easy way to include an error message for the 'Widget type' field on GET (so that it might show in the UI) but that may just be me missing something.

I also took a quick look at our DynamicTomSelect code and didn't see an immediate way to fix this there.

Either way, I think it's worth getting at least this fix in for the time being to remove a potentially very visible 500 error for users with better user feedback in a follow up fix. But, I'm open to other options. Thoughts?

[jeremystretch]

I think this is the right approach, but we can clean it up a bit by just ensuring that the initial value for the widget_class form field is always set to 'extras.NoteWidget' if not defined. We could change lines 1101-1103 to something like:

initial = {
    'widget_class': request.GET.get('widget_class') or 'extras.NoteWidget',
}

FWIW we should probably eliminate the "clear" button on required choice fields; I'll open a separate issue for that. (Edit: #18955)

[jeremystretch]

Thanks @jnovinger!