Free the private data pointer, THEN set it to NULL.

diff --git a/windows/x11drv/wnd.c b/windows/x11drv/wnd.c
index 7cd844d..703a534 100644
--- a/windows/x11drv/wnd.c
+++ b/windows/x11drv/wnd.c
@@ -112,8 +112,8 @@
 	  pWndDriverData->window
       );
     }
-  wndPtr->pDriverData = NULL;
   HeapFree(SystemHeap, 0, wndPtr->pDriverData);
+  wndPtr->pDriverData = NULL;
 }
 
 /**********************************************************************