How do I pass a structure through the vtk callback function for client data parameter?

Hello everybody. I have been trying to pass a struct of the following type:

struct ClientData {
    vtkSmartPointer<vtkCamera> camera;
    emscripten::val vtkCameraChangeFn;

Into the vtk callback functionality. In the following way:

 ClientData clientData = {.camera = this->renderer->GetActiveCamera(), .vtkCameraChangeFn = vtkCameraChangeFn};

    void *clientDataPointer;
    clientDataPointer = &clientData;

    // Set the callback to the function we created.

    // Set the client data.
    this->renderWindowInteractor->AddObserver(vtkCommand::InteractionEvent, callback);


However, the Web UI chrome debugger, I seem to be receiving the following error:

I mean, as far as I understand it, a parameter of type *void should have been able to accept a structure pointer. What is the error out here and how should it be overcome?