Undefined method failure_app for nil:NilClass - devise

Question

I am trying to use devise from a Rails app to grant access to a Sinatra application within the Rack middleware.
My config/routes.rb has:
authenticate "admin" do
mount Admins::Dashboard, :at => "/admins"
end
In my Sinatra app, I use:
before do
env["warden"].authenticate!(:scope => "admin")
end
get "/dashboard" do
erb :dashboard
end
Now, I get an error with the following stacktrace:
Started GET "/admins/dashboard" for 127.0.0.1 at 2012-10-11 08:45:13 +0200
NoMethodError (undefined method `failure_app' for nil:NilClass):
devise (2.1.2) lib/devise/delegator.rb:11:in `failure_app'
devise (2.1.2) lib/devise/delegator.rb:5:in `call'
warden (1.2.1) lib/warden/manager.rb:130:in `call_failure_app'
warden (1.2.1) lib/warden/manager.rb:116:in `process_unauthenticated'
warden (1.2.1) lib/warden/manager.rb:47:in `call'
actionpack (3.2.8) lib/action_dispatch/middleware/best_standards_support.rb:17:in `call'
rack (1.4.1) lib/rack/etag.rb:23:in `call'
rack (1.4.1) lib/rack/conditionalget.rb:25:in `call'

Answer 1

The error message is a complaint about missing configuration for customizing the warden failure app to point to sinatra.
Check out this previous answer and the related (detailed) blogpost about setting up Sinatra+Warden & Rails+Devise; hopefully the instructions there will help you implement the solution successfully.

Answer 2

Had the same error message and this comment on Github helped me out.
In my sessions_controller.rb I had:
sign_in(:user, #resource, store: false, bypass: false)
which I replaced with:
sign_in(:users, #resource, store: false, bypass: false)
for it to work.
Specifically, the issue was only happening after one user would delete his account... Also I am using devise_token_auth (hence the overriden sessions controller).