fix bug NoMethodError (undefined method `id' for nil:NilClass):
  vendor/bundle/ruby/1.9.1/bundler/gems/spree_simple_dash-d3df28ff397a/app/controllers/spree/admin/overview_controller.rb:118:in `best_selling_taxons'
  vendor/bundle/ruby/1.9.1/bundler/gems/spree_simple_dash-d3df28ff397a/app/controllers/spree/admin/overview_controller.rb:22:in `index'