"Could not resolve the actual object type" error when `null` returned in list of interfaces

[gao-artur]

Schema

type Query{
  test: [BaseType]
}
interface BaseType {
  name: ID!
}
type ConcreteType implements BaseType {
  name: ID!
}

Query

query {
  test {
    __typename
  }
}

Response

{
  "errors": [
    {
      "message": "Could not resolve the actual object type from `HotChocolate.Language.NullValueNode` for the abstract type `test`.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "test",
        0
      ]
    }
  ],
  "data": {
    "test": [
      null
    ]
  }
}

Expected behavior
null should be returned in list without errors.

Additional context
HC v11.2.2

[gao-artur]

Slack conversation
https://hotchocolategraphql.slack.com/archives/CD9TNKT8T/p1619428143159200

[PascalSenn]

Hi @gao-artur
I tried to reproduce the issue you described in #3626 but could not reporduce it in a test
cann you provide a repro of this issue?

[gao-artur]

Hi @PascalSenn
Was able to reproduce it only when using stitching. Please see repo here
https://github.com/gao-artur/graphql-stitching-3594

[gao-artur]

Hi @PascalSenn
I just tried to debug this issue and that's what I found:

When the remote schema executor reaches this line the result is null:

But when stitching schema executor reaches same line the result is NullValueNode:

Would this be the correct fix for the bug?

if (result is null || result is NullValueNode)
{
    completedResult = null;
    return true;
}

[michaelstaib]

this is fixed with 4064

[gao-artur]

@michaelstaib what version should I use to verify the fix? I just tried 12.0.0-rc.11 but the issue still reproduced.

[gao-artur]

@michaelstaib please reopen the bug, it still reproducible with 12.0.1.