cannot invoke a native callback outside of an isolate

We call CPP dll in Flutter using Flutter ffi. This is our CPP function in dll.

extern "C" __declspec(dllexport) void StartServerSession (const char* Name,const char* js,int& sessionID,int& ERESULT,void(eventhandler)(const char,int))
export "C"__declspec(dllexport) void InitializeServer(void(eventhandler)(const char,int)){ PtrObj->Initialize(eventhandler); }

This Flutter FFI code 

final DynamicLibrary DllPath = DynamicLibrary.open('path to dll');
   
typedef InitializeFunc = Void Function(Pointer<Void> eventhandler);
typedef InitializeFun = void Function(Pointer<Void> eventhandler);

typedef StartServerSessionC = Void Function(Pointer<Utf8> Name, Pointer<Utf8> js, Pointer<Pointer<Int32>> sessionID, Pointer<Pointer<Int32>> ERESULT,Pointer<Void> eventhandler);
typedef StartServerSession = void Function(Pointer<Utf8> Name, Pointer<Utf8> js, Pointer<Pointer<Int32>> sessionID, Pointer<Pointer<Int32>> ERESULT,Pointer<Void> eventhandler);

void eventHandler(Pointer<Utf8> response, int length) {
  print('Response: ${response.cast<Utf8>.toDartString()}');
}


final initialize = DllPath
    .lookupFunction<InitializeFunc , InitializeFun>('Initialize');

final startServerSession = DllPath
    .lookupFunction<StartServerSessionC, StartServerSession>('StartServerSession');


void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('FFI Demo')),
        body: Center(
          child: ElevatedButton(
            onPressed: () async {
              // Example usage of the FFI functions
                await callStartServerIsolate();
            },
            child: Text('Call DLL Function'),
          ),
        ),
      ),
    );
  }
}

void StartServerFunction(SendPort sendPort) 
{
   print("1");          
   Pointer<Utf8> name = ('path to second dll'.toNativeUtf8());
   Pointer<Utf8> js = {json value}.toNativeUtf8();
              
              // Allocate memory for sessionID and ERESULT
              final sessionID = calloc<Int32>();
          final ERESULT = calloc<Int32>();
            
              
              initialize (Pointer.fromFunction<Void Function(Pointer<Utf8>, Int32)>(eventHandler)as Pointer<Void>);

              // Run startOTXSession on a separate isolate
              startServerSession (
                               name,
                               js,
                               sessionID as Pointer<Pointer<Int32>>,
                               ERESULT as Pointer<Pointer<Int32>>, 
                               Pointer.fromFunction<Void Function(Pointer<Utf8>, Int32)>(eventHandler)as Pointer<Void>);
    
         sendPort.send("task Complete");
}

Future<Void> callStartServerIsolate()async{
   final receivePort = ReceivePort();
final sendPort = receivePort.sendPort;
await Isolate.spawn(StartServerFunction,sendPort)
}

** Showing an error that cannot invoke a native callback outside of an isolate.** Now what happened started SeverSession execution, **but internally they started a separate thread in the DLL file with a callback. **