Fix problem with product shipping category factory

spec/factories/product_factory.rb

@@ -18,7 +18,7 @@ FactoryBot.define do
     variant_unit_scale 1
     variant_unit_name ''
 
-    shipping_category { |r| Spree::ShippingCategory.first || r.association(:shipping_category) }
+    shipping_category { DefaultShippingCategory.find_or_create }
 
     # ensure stock item will be created for this products master
     before(:create) { create(:stock_location) if Spree::StockLocation.count == 0 }