Crash with Ember Source 6

[winsmith]

When using ember promise modals with ember-source 6, this error occurs when trying to open a modal:

Error occurred:

- While rendering:
  {{outlet}} for -top-level
    -top-level
      {{outlet}} for application
        application
          epm-modal-container
            epm-modal

[...]

Uncaught (in promise) Error: Attempted to resolve `action`, which was expected to be a helper, but nothing was found.

It seems to point at the use of "action" in epm-modal.hbs