Rails_admin permission on edit view of specific model

I am using rails_admin gem with rails 4. Currently I am using this method

# Authorize just admin for '/admin'
  config.authorize_with do
    redirect_to main_app.root_path unless warden.user.admin == true
  end

under config/initializers/rails_admin.rb to permit only the admin user to access the dashboard. PROBLEM: if the current user is admin and he/she is trying to change her permission to regular user (unticking) admin box then they get an error saying that no method for nil:admin - this is happening because /admin is no longer available for this regular user. QUESTION: How can I disallow the current user edit his own admin property ???