ActiveAdmin in rails engine give me that error : uninitialized constant MyEngine::Admin

0

I installed activeadmin in a rails engine that I mounted in my main application. When I try to access the home page I get the following error: uninitialized constant BackOffice::Admin

The code in lib/back_office/engine.rb

`require ‘devise’
require ‘activeadmin’

module BackOffice
class Engine < ::Rails::Engine
isolate_namespace BackOffice

initializer :back_office do
  ActiveAdmin.application.load_paths += Dir[File.dirname(__FILE__) + '/back_office/admin']
end

end
end
`

routes.rb
BackOffice::Engine.routes.draw do devise_for :admin_users, class_name: "BackOffice::AdminUser" ActiveAdmin.routes(self) end

In the routes.rb file of my main app I have this :
mount BackOffice::Engine, at: "/back_office"

Can someone help please ?