Hack to undecorate resource for forms. Regarding https://github.com/gregbell/active_admin/issues/2013

gistfile1.diff

 module ActiveAdmin
   module ViewHelpers
     module FormHelper

       def active_admin_form_for(resource, options = {}, &block)
         options = Marshal.load( Marshal.dump(options) )
         options[:builder] ||= ActiveAdmin::FormBuilder
+        resource = resource.respond_to?(:model) ? resource.model : resource
         semantic_form_for resource, options, &block
       end