Custom mutation throws: "Cannot read property 'args' of undefined"

Describe the bug Custom mutation throws the following error.

TypeError: Cannot read property 'args' of undefined
[run:server]     at ValidateCustomFieldsInterceptor.getArgumentMap (/app/node_modules/@vendure/core/src/api/middleware/validate-custom-fields-interceptor.ts:133:45)
[run:server]     at ValidateCustomFieldsInterceptor.intercept (/app/node_modules/@vendure/core/src/api/middleware/validate-custom-fields-interceptor.ts:50:45)
[run:server]     at /app/node_modules/@nestjs/core/interceptors/interceptors-consumer.js:23:36
[run:server]     at Object.handle (/app/node_modules/@nestjs/core/interceptors/interceptors-consumer.js:21:56)
[run:server]     at IdInterceptor.intercept (/app/node_modules/@vendure/core/src/api/middleware/id-interceptor.ts:38:21)
[run:server]     at /app/node_modules/@nestjs/core/interceptors/interceptors-consumer.js:23:36
[run:server]     at InterceptorsConsumer.intercept (/app/node_modules/@nestjs/core/interceptors/interceptors-consumer.js:25:24)
[run:server]     at target (/app/node_modules/@nestjs/core/helpers/external-context-creator.js:76:60)
[run:server]     at processTicksAndRejections (internal/process/task_queues.js:95:5)
[run:server]     at /app/node_modules/@nestjs/core/helpers/external-proxy.js:9:24

To Reproduce Steps to reproduce the behavior:

Define custom API extension (shop api)

extend type Mutation {
  doSomething: ActiveOrderResult!
}

1. Invoke this mutation in production (shopApiDebug: false in vendure config - doesnt seem to be happening locally when developing)
2. See error

Expected behavior No error. Seems this started with migration to Vendure 1.2.0 (worked before)

Environment (please complete the following information):

• @vendure/core version: 1.2.1
• Nodejs version: 14.17
• Database (mysql/postgres etc): postgres

[filipproch]

@michaelbromley I think I finally understood what is causing this. Its the proxy GraphQL which I am using (which is stitching two schemas together to create a single API- using @graphql-tools/stitch) a __typename everywhere.

Why it appeared now? I updated to the latest @graphql-tools version. Why I thought it appears randomly? I was testing it sometimes on the website, sometimes directly against the endpoint.

The solution could be to add check in the for loop, together with selection.kind === 'Field' to filter __typename out.

Should I submit a PR for this?

PS: sorry for spamming this thread - wanted to share the progress

[filipproch]

I did some changes to my GraphQL schema - so its probably connected to that rather than the version bump. I will try to debug it further and let you know here.