application_configs is kept hidden on Firefox

[yakky]

Description

Application configs dropdown by aldryn-apphooks-config does not work at least on firefox

## Steps to reproduce

A website with at least one application implementing aldryn-apphooks-config is required to trigger this
In the "advanced settings" section of a page, select an application which implements aldryn-apphooks-config

Expected behaviour

• application_namespaces text field is hidden
• application_configs select field is shown

Actual behaviour

• application_namespaces text field is kept displayed
• application_configs select field is kept hidden

Additional information

I can reproduce this at least in any 3.7 released version with any recent Firefox version (up to 76)

apparently this variable https://github.com/divio/django-cms/blob/develop/cms/static/cms/js/widgets/forms.apphookselect.js#L14 is not ready in firefox when that line is evaluated