trying to authenticate for rails_admin with devise

error

private method `current_user' called for #<RailsAdmin::MainController:0x007ffe52976be0>

rails_admin.rb

  1 RailsAdmin.config do |config|
  .
  .
  .
  6    config.authenticate_with do
  7      warden.authenticate! scope: :admin_user
  8    end
  9    config.current_user_method(&:current_user)
  .
  .
  .
  28  end