Undefined method 'assets' when loading the application

[texpert]

On Rails 3.2.21 I got:

Uncaught exception: There was an error while trying to load the gem 'activeadmin_hstore_editor'.
Gem Load Error is: undefined method `assets' for #<Rails::Engine::Configuration:0x007fa71b32d718>

Changing this line to an initializer, helps (think it is better to extract this to a separate initializer):

      initializer :active_admin_hstore_editor, :group => :all do |app|
        app.config.assets.precompile += %w[img/jsoneditor-icons.png]
      end

Besides, registering the javascript and stylesheets in the ActiveAdmin initializer doesn't properly load at least he stylesheets - had to require them as earlier, as usual.