Spree::Admin::VariantsController#index breaks for deleted products

[bbuchalter]

Steps to reproduce

chruby 2.4
git clone https://github.com/solidusio/solidus.git
cd solidus
bundle
bundle exec rake sandbox
cd sandbox
bin/rails s

• Visit http://localhost:3000/admin/products/ruby-on-rails-ringer-t-shirt/edit
• login as admin@example.com / test123
• Add the "Size" option type to the product and save
• Create a variant
• Return to the product admin index
• Search for "ringer"
• Delete the product
• Search for "ringer" with deleted and click on the product
• Go to the variants tab

Expected behavior

I should see the variants for the product

Actual behavior

NoMethodError (undefined method `variants' for nil:NilClass):

/Users/brian/workspace/solidus_master/backend/app/controllers/spree/admin/resource_controller.rb:189:in `collection'
/Users/brian/workspace/solidus_master/backend/app/controllers/spree/admin/variants_controller.rb:40:in `collection'
/Users/brian/workspace/solidus_master/backend/app/controllers/spree/admin/resource_controller.rb:144:in `load_resource'

System configuration

Solidus Version: master

Extensions in use: none