[This is preliminary documentation and subject to change.]
If your client application uses asynchronous event hooks (when the hook callback function is in your process and not the server's), it is possible that the window associated with an event no longer exists at the time the hook procedure is called. Therefore, you must verify that the window associated with an event still exists before taking any other action related to the event. To ensure that a window still exists, use the Win32 IsWindow function.
For more information about synchronous and asynchronous event hooks, see About the WinEventProc Callback Function.