Ensure Devise.available_router_name is never nil, closes #1648

lib/devise.rb

@@ -299,6 +299,10 @@ module Devise
     end
   end
 
+  def self.available_router_name
+    router_name || :main_app
+  end
+
   def self.omniauth_providers
     omniauth_configs.keys
   end

lib/devise/controllers/url_helpers.rb

@@ -60,7 +60,7 @@ module Devise
       private
 
       def _devise_route_context
-        @_devise_route_context ||= send(Devise.router_name)
+        @_devise_route_context ||= send(Devise.available_router_name)
       end
     end
   end

lib/devise/failure_app.rb

@@ -89,7 +89,7 @@ module Devise
       route = :"new_#{scope}_session_path"
       opts[:format] = request_format unless skip_format?
 
-      context = send(Devise.router_name)
+      context = send(Devise.available_router_name)
 
       if context.respond_to?(route)
         context.send(route, opts)

lib/devise/rails/routes.rb

@@ -9,13 +9,13 @@ module ActionDispatch::Routing
 
       @devise_finalized ||= begin
         if Devise.router_name.nil? && self != Rails.application.try(:routes)
-          warn "[Devise] We have detected that you are using devise_for inside engine routes. " \
+          warn "[DEVISE] We have detected that you are using devise_for inside engine routes. " \
             "In this case, you probably want to set Devise.router_name = MOUNT_POINT, where "   \
             "MOUNT_POINT is a symbol representing where this engine will be mounted at. For "   \
-            "now, Devise will default the mount point to :main_app."
+            "now Devise will default the mount point to :main_app. You can explicitly set it"   \
+            " to :main_app as well in case you want to keep the current behavior."
         end
 
-        Devise.router_name ||= :main_app
         Devise.configure_warden!
         Devise.regenerate_helpers!
         true