How to handle nullable reference in graphql

Question

Am new to grqphql/mongodb, I have a question which I didn't find a clear answer on the net, which is having the following schema (Nestjs/Graphql):

@ObjectType('CustomerServiceDetails')
@InputType('CustomerServiceDetailsInput')
@Schema({ timestamps: true })
export class CustomerServiceDetails {

  @Field(() => UpstreamProvider, { nullable: true })
  @Prop({
    type: mongoose.Schema.Types.ObjectId,
    ref: 'UpstreamProvider',
  })
  upstreamProvider?: UpstreamProvider;

  @Field(() => [ItpcConnection], { nullable: true })
  @Prop([{ type: mongoose.Schema.Types.ObjectId, ref: 'ItpcConnection' }])
  itpcConnections?: ItpcConnection;

}

Both of the properties are nullable which is working find during insert and update, however it returns error on Query "Cannot return null for non-nullable field Upstream Provider. name.", my question is how to handle nullable foreign keys in grqphql.

{
  customerServices {
    _id
    group
    customerServices {
      customerServiceDetails {
        upstreamProvider {
          name
        }
        itpcConnections {
          name
        }
      }
    }
  }
}

thanks