When creating a ui notification where the host is the subject and the host is owned by a Usergroup, this currently fails:

subject = host
audience is set to AUDIENCE_GROUP by app/services/ui_notifications/hosts.rb
When building the recipient_ids:
For a user: initiator.id is used
For a usergroup: subject.all_users is used, but not defined as subject is host.

I've written a test to demonstrate the behavior:

1) Error:
UINotificationsHostsTest::deliver notification to host owner#test_0002_owner is usergroup:
NoMethodError: undefined method `all_users' for #<Host::Managed:0x007f89bdbc91e0>
    app/models/notification.rb:49:in `subscriber_ids'
    app/models/notification.rb:65:in `set_notification_recipients'
    test_after_commit (1.1.0) lib/test_after_commit/database_statements.rb:11:in `block in transaction'
    test_after_commit (1.1.0) lib/test_after_commit/database_statements.rb:5:in `transaction'
    test/unit/ui_notifications/hosts/base_test.rb:7:in `create'
    app/services/ui_notifications.rb:26:in `deliver!'
    app/services/ui_notifications/hosts.rb:11:in `deliver!'
    test/unit/ui_notifications/hosts/base_test.rb:53:in `block (3 levels) in <class:UINotificationsHostsTest>'
    test/unit/ui_notifications/hosts/base_test.rb:52:in `block (2 levels) in <class:UINotificationsHostsTest>'