Change when file listeners are registered in order to make cache invalidation work

middleman-core/lib/middleman-core/sitemap/extensions/on_disk.rb

@@ -15,15 +15,17 @@ module Middleman
           scoped_self = self
           @waiting_for_ready = true
 
+          @app.before_configuration do
             # Register file change callback
-          @app.files.changed do |file|
+            files.changed do |file|
               scoped_self.touch_file(file)
             end
 
             # Register file delete callback
-          @app.files.deleted do |file|
+            files.deleted do |file|
               scoped_self.remove_file(file)
             end
+          end
 
           @app.ready do
             scoped_self.waiting_for_ready = false