Error message not correctly displayed

[dottxado]

I have customized the capabilities, removing the possibility of creating guest users, but the editor is showing an error on displaying the error:

I'm aware of #76, but this issue is about showing the right error message provided from the backend.

How to reproduce:

1. Add in a plugin a code like this

add_filter('user_has_cap', 'removeGuestAuthorCap', 9999);

function removeGuestAuthorCap(array $user_caps): array
    {
        unset($user_caps['create_guest_authors']);

        return $user_caps;
    }

2. Login to WordPress admin panel as an user that's not a Super Admin
3. Try to add a guest author to a post
4. The error message shown in the editor should report "undefined"