Dart: Close RecivePort from C finalizer - dart

Question

A dart object has a ReceivePort member, to be able to get notified from C (C++) code (Not necessarily from dart thread). When the object is collected by the GC, the port should be closed too, but it does not seem to do it automatically. Therefore, I attached a C finalizer to the object like this. First thing I tried is, to send a closing message (in my case just Null) to the port with Dart_PostCObject_DL, but the port doesn't get notified. I assume, that's by specification, because the finalizer should not call any dart code.
So is there another way of closing the port, when the object is being collected?
Failed Method:
// Dart code
class SomeClass {
ReceivePort? port;
Pointer<Void> nativeData;
SomeClass () {
// Code to create native SomeClassData Object
CLibrary.library.attachFinalizer(this, nativeData);
}
void addCallback(void Function(data) callback) {
if (port == null) {
port = ReceivePort();
CLibrary.library.addCallback(port!.sendPort.nativePort, nativeData);
port.listen((message) {
if(message == null) {
port!.close();
return;
}
callback(message);
}
}
}
}
// C++ code
struct SomeClassData {
uint64_t port;
}
static void runFinalizer(void* isolate_callback_data, void* peer) {
std::cout << "Running finalizer" << std::endl;
SomeClassData* data = static_cast<SomeClassData*>(peer);
if(data->port != 0) {
Dart_CObject obj;
obj.type = Dart_CObject_kNull;
Dart_PostCObject_DL(data->port, obj); // Returns false <=> Unsuccessful
}
}
void attachFinalizer(Dart_Handle handle, void* someClassDataPointer) {
Dart_NewFinalizableHandle_DL(handle, someClassDataPointer, sizeof(SomeClassData), runFinalizer);
}
void addCallback(uint64_t port, void* someClassDataPointer) {
static_cast<SomeClassData*(someClassDataPointer)->port = port;
// Do something with port
}