http://localhost:3000/graphql 실행 후 query > getHello 시 오류

http://localhost:3000/graphql 실행 후 query > getHello 시 오류가 있습니다.

코드는 오타 없이 동일 한데, 저는 실행시

http://localhost:3000/graphql 여기서 아래와 같은 오류가 뜨는데 어떤 상황인지 파악이 잘 되지가 않습니다.

{
  "errors": [
    {
      "message": "Cannot return null for non-nullable field Query.getHello.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
      "path": [
        "getHello"
      ],
      "extensions": {
        "code": "INTERNAL_SERVER_ERROR",
        "exception": {
          "stacktrace": [
            "Error: Cannot return null for non-nullable field Query.getHello.",
            "    at completeValue (/Users/back/study/20230119_nestjs/class/13-01-nestjs-with-graphql/node_modules/graphql/execution/execute.js:594:13)",
            "    at /Users/back/study/20230119_nestjs/class/13-01-nestjs-with-graphql/node_modules/graphql/execution/execute.js:486:9",
            "    at processTicksAndRejections (node:internal/process/task_queues:95:5)",
            "    at async Promise.all (index 0)",
            "    at execute (/Users/back/study/20230119_nestjs/class/13-01-nestjs-with-graphql/node_modules/apollo-server-core/src/requestPipeline.ts:501:14)",
            "    at processGraphQLRequest (/Users/back/study/20230119_nestjs/class/13-01-nestjs-with-graphql/node_modules/apollo-server-core/src/requestPipeline.ts:407:22)",
            "    at processHTTPRequest (/Users/back/study/20230119_nestjs/class/13-01-nestjs-with-graphql/node_modules/apollo-server-core/src/runHttpQuery.ts:436:24)"
          ]
        }
      }
    }
  ],
  "data": null
}

app.module.ts 에도 정상적으로 연결 해 두었습니다.

import { Module } from '@nestjs/common'
// import { AppController } from './app.controller';
// import { AppService } from './app.service';
import { BoardsModule } from './apis/boards/boards.module'
import { GraphQLModule } from '@nestjs/graphql'
import { ApolloDriver, ApolloDriverConfig } from '@nestjs/apollo'

@Module({
  imports: [
    BoardsModule,
    GraphQLModule.forRoot<ApolloDriverConfig>({
      driver: ApolloDriver,
      autoSchemaFile: 'src/commons/graphql/schema.gql',
    }),
  ],
  // controllers: [AppController],
  // providers: [AppService],
})
export class AppModule {}