Improve Related Documents plugin configuration screen

Can't add Related Documents field to document types using the Related Documents plugin in Essentials, because the 'Add related documents' button is disabled.

Reproduction path:

In the latest archetype, based on 12.0.2:

-Add Banners & News features via Essentials

-Add Related Documents plugin

-Rebuild & restart

-Go to Related Documents plugin in Essentials

-Select one of the doctypes, e.g. banner, the button on bottom right 'Add related documents' is disabled which prevents the users from being able to add the field to any document types.