`admin/bin/rails generate godmin:install` fails: "admin/application.js does not appear to exist"

[dgmstuart]

While following the setup instructions, and running admin/bin/rails generate godmin:install on a Rails 6 application, I get the following error:

The file ~/dev/my_app/admin/app/assets/javascripts/admin/application.js does not appear to exist

[dgmstuart]

This is because in Rails 6 this has moved to app/javascript/packs/application.js

[dgmstuart]

Or... actually the rails engine no longer generates any javascript assets at all...