Fix NullReferenceException

Fix NullReferenceException in FileSystemWatcher on Windows if two threads try and dispose the instance at the same time.

src/libraries/System.IO.FileSystem.Watcher/src/System/IO/FileSystemWatcher.Win32.cs

@@ -105,7 +105,7 @@ private void StopRaisingEvents()
             // increased, such that the disposal operation won't take effect and close the handle
             // until that P/Invoke returns; if during that time the FSW is restarted, the IsHandleInvalid
             // check will see a valid handle, unless we also null it out.
-            _directoryHandle.Dispose();
+            _directoryHandle?.Dispose();
             _directoryHandle = null;
         }