Complains about null for non-null type

[rburgst]

I am getting the following error message:

Cannot return null for non-nullable field CMCategory.nameDe.

However, none of the returned CMCategory from the origin graphql server returns null for this field.

A reproduction sample is available at
https://github.com/rburgst/gatsby-graphql-toolkit-bug

This is the server schema

  type Query {
    getCategories: [Category]!
    getConcerts: [Concert]!
  }

  type Category {
    id: String!
    nameDe: String!
    nameEn: String!
    displayOrder: Int!
    isSubcategoryOf: Category
  }
  type Concert {
    id: ID!
    category: Category
    secondCategory: Category
    titleDe: String!
    titleEn: String!
  }

To replicate do the following

1. start the dev server

   cd server
   pnpm install
   pnpm dev
   

2. In another shell start a gatsby build

   cd gatsby
   yarn install
   yarn build
   

Expected

the build runs through

Actual

The following error is thrown

 ERROR #85925  GRAPHQL

There was an error in your GraphQL query:

Cannot return null for non-nullable field CMCategory.nameDe.

The field "CMCategory.nameDe." was explicitly defined as non-nullable via the schema customization API (by yourself or a plugin/theme). This means that this field is not optional and you have to define a value. If this is not your
desired behavior and you defined the schema yourself, go to "createTypes" in gatsby-node.js. If you're using a plugin/theme, you can learn more here on how to fix field types:
https://www.gatsbyjs.com/docs/reference/graphql-data-layer/schema-customization#fixing-field-types

   1 | fragment ConcertCategoryFragment on CMCategory {
   2 |   remoteId
>  3 |   nameDe
     |   ^
   4 |   nameEn
   5 |   isSubcategoryOf {
   6 |     remoteId
   7 |     nameDe
   8 |     nameEn
   9 |   }
  10 | }
  11 |

[rburgst]

Note that when I run the following graphql query in the gatsby graphiql

query MyQuery {
  allCmConcert {
    nodes {
      id
      titleDe
      titleEn
      category {
        remoteId
        nameDe
        isSubcategoryOf {
          remoteId
          nameDe
        }
      }
    }
  }
}

I get the 2nd concert with a null link:

       [ 
       {...},
        {
          "id": "Concert:69e2025a-2573-4d25-b2e1-a6d34a7bcefd",
          "titleDe": "fgdg",
          "titleEn": "asdf",
=>           "category": null
        }
      ]
    }

Even though the source server returns a valid category id.