Reflection AddMessage cause pure vritual method called and crash

[uni x]

I'm parsing message by runtime proto-file. I constructed "prototye message", and tried to get reflection infos of each fields. when it comes to nested message, I need to `AddMessage` to get the inside fields.

// proto-file

```

message A {

   int32  a1 = 1;

   string a2 = 2;

};

message B {

   repeated A b1 = 1;

   int64      b2 = 2;

};

```

```

Class ProtoType {

Message* msg;

BizX* biz_x;

void Init(const std::string& proto_file_path) {

    // parse from file to get msgDesc

    auto* factory = new google::protobuf::DynamicMessageFactory();

    factory->SetDelegateToGeneratedFactory(true);                                                                 

    msg= mFactory->GetPrototype(msgDesc);         

}

void InitRefl() {

    biz_x = new BizX();

    biz_x->Init(*(msg->New()));

}

};

```

```

class BizX {

BizY* biz_y;

Reflection* refl;

Descriptor* desc;

void Init(const Message& msg) {

    refl = msg.GetReflection();

    desc = msg.GetDescriptor();

    const auto* field_desc = desc->FindFieldByName("b1");

    auto* p = msg.New();

    auto* tmp = refl->AddMessage(p, field_desc);

    biz_y = new BizY();

    biz_y->Init(msg);

} 

};

```

```

class BizY {

Reflection* refl;

Descriptor* desc;

void Init(const Message& msg) {

    refl = msg.GetReflection();

    desc = msg.GetDescriptor();

    const auto* field_desc = desc->FindFieldByName("b1");

    auto* p = msg.New();

    auto* tmp = refl->AddMessage(p, field_desc); // crash!!!:pure virtual method called

}

};

```

```

int main() {

    ProtoType* p = new ProtoType();

    p->Init("proto-file")

    p->InitRefl();

    return 0;

}

```

I thought `BizY::Init` should have the same result as `BizX::Init`, but it crashed, why?