GraphQL Error replication

This is a repo to provide a test-case for silverstripe/silverstripe-graphql#468

Installation

Clone this repo and cd into the directory. Then do:

docker-compose up -d
./do.sh composer install
./do.sh sake dev/build

This will start a SilverStripe Webserver on http://localhost:8000

Reproduce issue

Go to the GraphQL IDE: http://localhost:8000/dev/graphql/ide

Run the following query and see how it fails:

query {
  readPages {
    nodes {
      title
      __typename
      ... on CustomPage {
        customField
      }
    }
  }
}

If you remove the extension from app/_config/extensions.yml, then do a dev/build followed by the query above, it will work (since the bulkLoad API did not load anything).