Selecting selected_shipping_methods when there's none configured results in an Internal Server Error

[dplaton]

Preconditions (*)

1. Magento 2.3.2
2. PHP 7.2

Steps to reproduce (*)

1. Create a guest cart using the createGuestCart mutation
2. Add a shipping address to the cart
3. Perform the following query:

query {
  cart(cart_id: "your cart id") {
    shipping_addresses {
  	available_shipping_methods {
      carrier_code
    }
      selected_shipping_method {
        carrier_code
        carrier_title
        method_code
        method_title
      }
    }
  }
}

Expected result (*)

1. The selected shipping method is empty/null
2. The available shipping methods object to contain the correct carrier codes configured

Actual result (*)

An error is returned:

{
  "errors": [
    {
      "message": "Internal server error",
      "category": "internal",
      "locations": [
        {
          "line": 7,
          "column": 7
        }
      ],
      "path": [
        "cart",
        "shipping_addresses",
        0,
        "selected_shipping_method"
      ]
    }
  ],
  "data": {
    "cart": {
      "shipping_addresses": [
        {
          "available_shipping_methods": [
            {
              "carrier_code": "flatrate"
            }
          ],
          "selected_shipping_method": null
        }
      ]
    }
  }
}

Note that this error does not occur when we only select the selected_shipping_methods (without the available_shipping_methods), so a query like

query {
  cart(cart_id: "Kuvi8t2mSaqeh8pkWBnLXXmqQnkLu0Tr") {
    shipping_addresses {
      selected_shipping_method {
        carrier_code
        carrier_title
        method_code
        method_title
      }
    }
  }
}

returns the expected result.

This issue looks a lot like #665 and according to (#665 (comment)) it should've been investigated. However, I couldn't find and PR to fix it.

[lenaorobei]

In progress by internal team.

[lenaorobei]

Fixed.