I am creating a windows application. I am trying to set up an Application class and handle events in a member function inside it, and have an instance of the class declared beforehand to be called inside the windows procedure.

I expect seeing a white unresponsive window but the window creation returns nullptr. This is the main.cpp

#include <windows.h>
#include "WinApplication.h"

WinApplication* app;

LRESULT CALLBACK windowCallback(HWND hwnd, UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    //return DefWindowProc(hwnd, uMsg, wParam, lParam);
    return app->appCallback(uMsg, wParam, lParam);
}

int WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd)
{
    app = new WinApplication();
    WNDCLASS window_class = {};
    window_class.style = CS_HREDRAW | CS_VREDRAW;
    window_class.lpszClassName = TEXT("Window Class");
    window_class.lpfnWndProc = windowCallback;
    
    // Register Class
    if (!RegisterClass(&window_class))
        return 1;
    
    // Create Window
    HWND window = CreateWindow(window_class.lpszClassName, TEXT("MyApp"), WS_OVERLAPPEDWINDOW | WS_VISIBLE, CW_USEDEFAULT, CW_USEDEFAULT, DEF_WIDTH, DEF_HEIGHT, 0, 0, hInstance, 0);
    
    app->init(window);
    while (true) {  }

    return 0;
}

and this is the member callback:

LRESULT WinApplication::appCallback(UINT uMsg, WPARAM wParam, LPARAM lParam)
{
    return 0;
}

Any help would be appreciated