show a button above flutter window twice but exit with Callback invoked after it has been deleted

[Ferry-200]

I am making a floating lyric window which can display lyric text above all windows.

In a IconButton, I execute this code to make the window to be transparent. It make the window ignore mouse event.

final hWnd = win32.GetForegroundWindow();

            final exStyle = win32.GetWindowLongPtr(
              hWnd,
              win32.WINDOW_LONG_PTR_INDEX.GWL_EXSTYLE,
            );

            win32.SetWindowLongPtr(
              hWnd,
              win32.WINDOW_LONG_PTR_INDEX.GWL_EXSTYLE,
              exStyle |
                  win32.WINDOW_EX_STYLE.WS_EX_LAYERED |
                  win32.WINDOW_EX_STYLE.WS_EX_TRANSPARENT,
            );

            Isolate.spawn<int>(showUnlockOverlay, hWnd);

But I also want to provide a way to make the window to be not transparent. This is why I want to show a button above this flutter window.

int mainWindowProc(int hWnd, int uMsg, int wParam, int lParam) {
  switch (uMsg) {
    case win32.WM_CREATE:
      win32.CreateWindow(
        win32.TEXT("BUTTON"),
        win32.TEXT("UNLOCK"),
        win32.WINDOW_STYLE.WS_VISIBLE | win32.WINDOW_STYLE.WS_CHILD,
        10,
        10,
        80,
        30,
        hWnd,
        1,
        win32.GetWindowLongPtr(
          hWnd,
          win32.WINDOW_LONG_PTR_INDEX.GWLP_HINSTANCE,
        ),
        nullptr,
      );
      break;
    case win32.WM_COMMAND:
      if (win32.LOWORD(wParam) == 1) {
        // make the flutter window to be not transparent
        final parentHwnd = win32.GetWindowLongPtr(
          hWnd,
          win32.WINDOW_LONG_PTR_INDEX.GWLP_HWNDPARENT,
        );
        final exStyle = win32.GetWindowLongPtr(
          parentHwnd,
          win32.WINDOW_LONG_PTR_INDEX.GWL_EXSTYLE,
        );
        win32.SetWindowLongPtr(
          parentHwnd,
          win32.WINDOW_LONG_PTR_INDEX.GWL_EXSTYLE,
          exStyle &
              ~win32.WINDOW_EX_STYLE.WS_EX_LAYERED &
              ~win32.WINDOW_EX_STYLE.WS_EX_TRANSPARENT,
        );
        win32.DestroyWindow(hWnd);
      }
      break;
    case win32.WM_DESTROY:
      win32.PostQuitMessage(0);
      break;
    default:
      return win32.DefWindowProc(hWnd, uMsg, wParam, lParam);
  }
  return 0;
}

void showUnlockOverlay(int parentHwnd) {
  // Register the window class.
  final className = win32.TEXT('desktop_lyric_unlock_overlay');

  final lpfnWndProc = NativeCallable<win32.WNDPROC>.isolateLocal(
    mainWindowProc,
    exceptionalReturn: 0,
  );

  final hInstance = win32.GetModuleHandle(nullptr);

  final wc = ffi.calloc<win32.WNDCLASS>()
    ..ref.lpfnWndProc = lpfnWndProc.nativeFunction
    ..ref.hInstance = hInstance
    ..ref.lpszClassName = className;
  win32.RegisterClass(wc);

  // Create the window.
  final windowCaption = win32.TEXT('desktop_lyric_unlock_overlay');
  final hWnd = win32.CreateWindowEx(
    win32.WINDOW_EX_STYLE.WS_EX_TOPMOST |
        win32.WINDOW_EX_STYLE.WS_EX_LAYERED |
        win32.WINDOW_EX_STYLE.WS_EX_TOOLWINDOW,
    className, // Window class
    windowCaption, // Window caption
    win32.WINDOW_STYLE.WS_POPUP, // Window style

    // Size and position
    0,
    0,
    100,
    50,
    parentHwnd, // Parent window
    win32.NULL, // Menu
    hInstance, // Instance handle
    nullptr, // Additional application data
  );
  win32.free(windowCaption);
  win32.free(className);

  if (hWnd == 0) {
    final error = win32.GetLastError();
    throw win32.WindowsException(win32.HRESULT_FROM_WIN32(error));
  }

  // set the opacity to 70%
  win32.SetLayeredWindowAttributes(
    hWnd,
    0,
    (255 * 70) ~/ 100,
    win32.LAYERED_WINDOW_ATTRIBUTES_FLAGS.LWA_ALPHA,
  );

  win32.ShowWindow(hWnd, win32.SHOW_WINDOW_CMD.SW_SHOW);

  // locate this button window to flutter's topleft
  final targetRect = ffi.calloc<win32.RECT>();
  win32.GetWindowRect(parentHwnd, targetRect);
  win32.SetWindowPos(
    hWnd,
    win32.HWND_TOPMOST,
    targetRect.ref.left,
    targetRect.ref.top,
    100,
    50,
    win32.SET_WINDOW_POS_FLAGS.SWP_SHOWWINDOW,
  );
  ffi.calloc.free(targetRect);

  // Run the message loop.
  final msg = ffi.calloc<win32.MSG>();
  while (win32.GetMessage(msg, win32.NULL, 0, 0) != 0) {
    win32.TranslateMessage(msg);
    win32.DispatchMessage(msg);
  }

  lpfnWndProc.close();
  win32.free(msg);
}

All work for the first time. But the 2nd time I press the button, the flutter window exit with the error:

../../third_party/dart/runtime/vm/runtime_entry.cc: 4089: error: Callback invoked after it has been deleted.
version=3.4.0 (stable) (Mon May 6 07:59:58 2024 -0700) on "windows_x64"
pid=23748, thread=388, isolate_group=main(000001FDD91C4250), isolate=showUnlockOverlay(000001FDF5722840)
os=windows, arch=x64, comp=no, sim=no
isolate_instructions=7fff242763d0, vm_instructions=7fff242763e0
fp=2fc48fe528, sp=2fc48fe528, pc=7fff240674a2
  pc 0x00007fff240674a2 fp 0x0000002fc48fe528 InternalFlutterGpu_Texture_AsImage+0x12b4ca
  pc 0x0000002fc48fe688 fp 0x0000002fc48fe698 Unknown symbol
-- End of DumpStackTrace
  pc 0x0000000000000000 fp 0x0000002fc48ff070 sp 0x0000000000000000 Cannot find code object
  pc 0x000001fde7b6f548 fp 0x0000002fc48ff100 sp 0x0000002fc48ff080 [Optimized] init:_CreateWindowEx@277426514.#ffiClosure56
pc 0x000001fde7b6f191 fp 0x0000002fc48ff198 sp 0x0000002fc48ff110 [Unoptimized] CreateWindowEx
  pc 0x000001fde7b691f4 fp 0x0000002fc48ff268 sp 0x0000002fc48ff1a8 [Unoptimized] showUnlockOverlay
  pc 0x000001fde8572e89 fp 0x0000002fc48ff298 sp 0x0000002fc48ff278 [Unoptimized] showUnlockOverlay
  pc 0x000001fddcda707d fp 0x0000002fc48ff300 sp 0x0000002fc48ff2a8 [Unoptimized] [email protected]:call
  pc 0x000001fddcda7384 fp 0x0000002fc48ff348 sp 0x0000002fc48ff310 [Unoptimized] _delayEntrypointInvocation@1026248.<anonymous closure>
  pc 0x000001fddcda707d fp 0x0000002fc48ff3b0 sp 0x0000002fc48ff358 [Unoptimized] [email protected]:call
  pc 0x000001fddcda688c fp 0x0000002fc48ff408 sp 0x0000002fc48ff3c0 [Unoptimized] _RawReceivePort@1026248._handleMessage@1026248
  pc 0x000001fdd9702f9f fp 0x0000002fc48ff530 sp 0x0000002fc48ff418 [Stub] InvokeDartCode

And here is a video.

2024-08-06.11-30-44.mp4

Please, can someone tell me what's wrong or how to debug this. Thanks!