为什么 MessageBox 不显示？

为什么在关闭程序之前没有出现MessageBox- 在窗口过程完成并观察其调用条件之后？只有在之前调用它时才会出现PostQuitMessage。

#include <windows.h>
//#include <cstdio>

long __stdcall WindowProc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam) {
    if (uMsg == WM_CLOSE)
        PostQuitMessage(0);
    else if (!FindWindowExA(nullptr,nullptr,"TargetAppClass",nullptr))
        PostQuitMessage(1);
    return DefWindowProcA(hWnd, uMsg, wParam, lParam);
}

int __stdcall WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, PSTR lpCmdLine, int nCmdShow) {
    (void) hPrevInstance;
    (void) lpCmdLine;
    (void) nCmdShow;

    WNDCLASSEXA WinClass;
    memset(&WinClass,0,sizeof(WNDCLASSEXA));
    WinClass.cbSize = sizeof(WNDCLASSEXA);
    WinClass.hInstance = hInstance;
    WinClass.lpfnWndProc = WindowProc;
    WinClass.lpszClassName = "MonitoringApp";

    CreateMutexA(NULL, TRUE, WinClass.lpszClassName);
    if (ERROR_ALREADY_EXISTS == GetLastError()) return ERROR_ALREADY_EXISTS;

    RegisterClassExA(&WinClass);
    auto win = CreateWindowExA(0,WinClass.lpszClassName,WinClass.lpszClassName,0,0,0,0,0,nullptr,nullptr,WinClass.hInstance,nullptr);
    MSG msg;
    while(GetMessageA(&msg,win,0,0)) {
        TranslateMessage(&msg);
        DispatchMessageA(&msg);
    }
//    printf("%u, %u\n",msg.message,msg.wParam); // показывает 18, 1 (WM_QUIT и код выхода)
    if (msg.wParam) // MessageBox не выводится. Почему?
        MessageBoxA(nullptr, "Seems that the TargetApp crashed", "Monitoring App", MB_ICONWARNING | MB_OK | MB_SYSTEMMODAL);
    return 0;
}