Nested/2D arrays don't work with union types

[Thomas Grice]

Describe the Bug
If you use a union type within a nested array, you'll see an error like this:

<REPO_ROOT>/node_modules/type-graphql/build/cjs/schema/schema-generator.js:69
return unionObjectTypesInfo.map(it => it.type);
TypeError: Cannot read properties of undefined (reading 'type')

To Reproduce

1. Create enum type
2. Reference it inside of a 2-dimensional (nested) array inside of an ObjectType

export const SomeUnionType = createUnionType({
  name: 'SomeUnionType',
  types: () => [String, SomeType, SomeOtherType] as const,
  resolveType: (value) => {
    // type resolution logic here
  },
})

@ObjectType()
export class SomeOutput {
  // Project runs/works if I comment this out:
  @Field((type) => [[SomeUnionType]])
  rows: SomeUnionType[][]
}

Expected Behavior
I expect this to work/not crash the app.

Logs
If applicable, add some console logs to help explain your problem.
You can paste the errors with stack trace that were printed when the error occurred.

Environment (please complete the following information):

• OS: MacOS
• Node 16.20.2
• Type-graphql 2.0.0-beta.3
• TypeScript version 4.x

Additional Context
This makes making typed grid/table content difficult.

[Thomas Grice]

Oops, I think I messed up here and the problem is the use of String in a union type, and not the union type otherwise. Maybe have a better error message here?

[Michał Lytek]

Yes, the problem is with String type.

Can you add your suggestion about better error message in #12? 🙏