Observer + Notifier give "Unprocessed view path found: nil"

I have a UserObserver registered which handles sending the emails, via
a UserNotifier, for user signup etc.

It works fine in my specs (well everything is being called as
expected) however in the application if I call I get the
following error:

"Unprocessed view path found: nil. Set your view paths with
#append_view_path, #prepend_view_path, or #view_paths=."

I've tried adding the following to my mailer:

ActionController::Base.view_paths << Rails.root + '/app/views'

But that doesn't make any difference, I guess the observer is outside
of the realm of the controller. I'm sure this was working a while back
but doesn't seem to be anymore (and I've made a lot of changes since

Any ideas?

I figured out that this was an issue with a couple of plugins I have

I'm using ARMailer to send the emails and I had also installed the
exception_notification plugin. It seems to be the
exception_notification plugin that causes the issue and my only
solution was to disable this plugin for now (I have logged this issue