TypeGraphQL Generator with Union type fails with error

Describe the bug

When generating TypeGraphQL, using a union type, the type passed to @TypeGraphQL.Field fails with the error: 'Step' only refers to a type, but is being used as a value here. (TS 2693)

To Reproduce Steps to reproduce the behavior:

Run the generation below.

1. My GraphQL schema:

type PullStep @entity {
  nextSteps: [Step!] @column
  # ...other fields...
}

type PushStep @entity {
  nextSteps: [Step!] @column
  # ...other fields...
}

union Step = PullStep | PushStep

2. My codegen.yml config file:

generates:
  types.ts:
    plugins:
      - typescript-type-graphql

3. The output:

import * as TypeGraphQL from 'type-graphql';
export { TypeGraphQL };
export type Maybe<T> = T | null;
export type Exact<T extends { [key: string]: unknown }> = { [K in keyof T]: T[K] };
export type MakeOptional<T, K extends keyof T> = Omit<T, K> & { [SubKey in K]?: Maybe<T[SubKey]> };
export type MakeMaybe<T, K extends keyof T> = Omit<T, K> & { [SubKey in K]: Maybe<T[SubKey]> };
export type FixDecorator<T> = T;
/** All built-in and custom scalars, mapped to their actual values */
export type Scalars = {
  ID: string;
  String: string;
  Boolean: boolean;
  Int: number;
  Float: number;
};

@TypeGraphQL.ObjectType()
export class PullStep {
  __typename?: 'PullStep';

  @TypeGraphQL.Field(type => [Step], { nullable: true })
  // ^^^^ !!! "'Step' only refers to a type, but is being used as a value here. (TS 2693)" !!!
  // ^^^^
  nextSteps!: Maybe<Array<Step>>;
};

@TypeGraphQL.ObjectType()
export class PushStep {
  __typename?: 'PushStep';

  @TypeGraphQL.Field(type => [Step], { nullable: true })
  // ^^^^ !!! "'Step' only refers to a type, but is being used as a value here. (TS 2693)" !!!
  // ^^^^
  nextSteps!: Maybe<Array<Step>>;
};

export type Step = PullStep | PushStep;

Expected behavior

No error. To fix this, it probably needs a union type generated: https://typegraphql.com/docs/unions.html

Environment:

• OS: Linux
• @graphql-codegen/...: 1.21.3
• NodeJS: 14

Additional context