Cannot return null for non-nullable field SelectedShippingMethod.amount

[TomashKhamlai]

Cannot return null for non-nullable field SelectedShippingMethod.amount when querying "selected_shipping_method" on "setShippingAddressesOnCart" mutation no matter shipping_addresses.address or shipping_addresses.customer_address_id is used

Preconditions (*)

1. Registered Customer
2. Simple Product
3. Virtual Product
4. Developer mode

Steps to reproduce (*)

1. Generate Customer token
2. Create empty cart as Registered Customer from GraphQL Client
3. Add simple and virtual products to cart from GraphQL Client but use

   ...
   cart {
     items {
       id
       __typename
   ...

   in output

4. Get ADDRESS_ID by hovering 'Change Billing Address' or 'Change Shipping Address'
5. Set

   mutation {
   setShippingAddressesOnCart(
   input: {
     cart_id: "{{ CART_ID }}"
     shipping_addresses: {
       customer_address_id: 3
       cart_items: [{
         cart_item_id: 11
         quantity: 1
       }
       {
         cart_item_id: 12
         quantity: 1
       }]
     }
   }
   ) {
   cart {
     cart_id
     shipping_addresses {
       address_type
       available_shipping_methods {
         carrier_code
         carrier_title
         method_code
         method_title
         price_excl_tax
         price_incl_tax
       }
       selected_shipping_method {
         amount
       }
     }
   }
   }
   }

Expected result (*)

1. Some tip for guessing what is wrong

Actual result (*)

1. "Cannot return null for non-nullable field SelectedShippingMethod.amount.

   {
   "errors": [
   {
     "debugMessage": "Cannot return null for non-nullable field SelectedShippingMethod.amount.",
     "message": "Internal server error",
     "category": "internal",
     "locations": [
       {
         "line": 16,
         "column": 11
       }
     ],
     "path": [
       "setShippingAddressesOnCart",
       "cart",
       "shipping_addresses",
       0,
       "selected_shipping_method",
       "amount"
     ]
   }
   ],
   "data": {
   "setShippingAddressesOnCart": {
     "cart": {
       "cart_id": "rzQ6tNveErjEWMWzDJMVlnfgtkhiVPOu",
       "shipping_addresses": [
         {
           "address_type": null,
           "available_shipping_methods": [
             {
               "carrier_code": "flatrate",
               "carrier_title": "Flat Rate",
               "method_code": "flatrate",
               "method_title": "Fixed",
               "price_excl_tax": 10,
               "price_incl_tax": 10
             }
           ],
           "selected_shipping_method": null
         }
       ]
     }
   }
   }
   }

[naydav]

https://github.com/magento/graphql-ce/pull/417