magento2: "Fatal error: Uncaught TypeError" in Cart when Flat Catalog is enabled and Quote contains disabled Configurable product

if persistent cart is enabled, a user’s quote might contain a product that is now disabled in the catalog. if this happens when Flat Product Catalog is enabled, the cart page will throw a Fatal Error, and the entire cart functionality will not be usable for this customer until the quote is manually deleted from the database.

Preconditions (*)

1. 2.2.7 CE

Steps to reproduce (*)

backend:

1. activate persistent cart
2. activate flat product catalog
3. create configurable product SKU parent
4. create at least one child configuration item with SKU child
5. reindex in the cli: indexer:reindex catalog_product_flat

frontend:

1. create new customer & login
2. add configurable product child to cart
3. log out

backend:

1. set status of child to disabled

frontend:

1. login as customer
2. open cart

Expected result (*)

a) cart is showing product child but as “not available anymore” b) cart is empty

Actual result (*)

• cart page isn’t showing the product
• error is logged
• no further product can be added to the cart
• checkout fails

Fatal error: Uncaught TypeError: Return value of Magento\ConfigurableProduct\Model\Product\Configuration\Item\ItemProductResolver::getChildProduct() must be an instance of Magento\Catalog\Model\Product, null returned in /var/www/users/demo/src/www/vendor/magento/module-configurable-product/Model/Product/Configuration/Item/ItemProductResolver.php:83
Stack trace: 
#0 /var/www/users/demo/src/www/vendor/magento/module-configurable-product/Model/Product/Configuration/Item/ItemProductResolver.php(51): Magento\ConfigurableProduct\Model\Product\Configuration\Item\ItemProductResolver->getChildProduct(Object(Magento\Quote\Model\Quote\Item\Interceptor)) 
#1 /var/www/users/demo/src/www/vendor/magento/module-catalog/Model/Product/Configuration/Item/ItemResolverComposite.php(44): Magento\ConfigurableProduct\Model\Product\Configuration\Item\ItemProductResolver->getFinalProduct(Object(Magento\Quote\Model\Quote\Item\Interceptor)) 
#2 /var/www/users/demo/src/www/vendor/magento/module-checkout/Block/Cart/Item/Re in /var/www/users/demo/src/www/vendor/magento/module-configurable-product/Model/Product/Configuration/Item/ItemProductResolver.php on line 83

2.2.7 added an explicit return type in ItemProductResolver::getChildProduct() which doesn’t allow returning null: private function getChildProduct(ItemInterface $item): Product. while this is the reason for the fatal error, the underlying problem seems to be that the flat catalog doesn’t contain disabled items and thus will return null for disabled products.