我正在使用gRPC通信，同时调用其他微服务模块中的服务。
下面是我的示例.proto文件

syntax = "proto3";
import public "common/session-context.proto";
import public "common/serviceInvocationModeType.proto";
import public "common/auditable.proto";
import public "common/coreEngineBase.proto";
import public "common/transactionStatus.proto";
import public "common/date.proto";
option java_multiple_files = true;
option java_package = "com.mnp.grpc.casa";
message CasaAccountOpeningGrpcDTO{
  string accountTitle = 1;
  string systemStatus = 2;
  AccountTypeEnum accountType = 3;
}
message CasaAccountOpeningGrpcResponseDTO{
  string accountNumber = 1;
  string applicationId = 2;
}
message CasaAccountOpeningRequest{
    SessionContext sessionContext = 1;
    CasaAccountOpeningGrpcDTO casaAccountOpeningGrpcDTO = 2;
}
message CasaAccountOpeningResponse{
  CasaAccountOpeningGrpcResponseDTO casaAccountOpeningGrpcResponseDTO = 1;
  TransactionStatus transactionStatus = 2;
}
enum AccountTypeEnum{
  CASA = 0;
  ABC = 1;
  XYZ = 2;
}
service CasaAccountOpeningService{
rpc processAccountOpening(CasaAccountOpeningRequest) returns (CasaAccountOpeningResponse) ;
}

字符串
现在，每当我想在调用服务时将AccountTypeEnum值作为null发送时，如果我将AccountTypeEnum作为null传递，那么它会给出一个指针异常。如何在使用gRPC服务时将AccountTypeEnum作为null传递？