Fix NPE in WatchQueueReader

[jimtng]

Fix NPE in WatchQueueReader. The NPE would cause the watch queue to stop being watched, and script files will no longer be reloaded when changed.

This seems to occur when files were created and quickly removed within the watched directories. It should resolve #2537 and #2535

[wborn]

Thanks for fixing the NPE! It is also identified by the null analysis. I think it's a good idea to do the null check because there can be all sorts of native implementation specific weirdness going on after reading the WatchService JavaDocs:

 * <p> The implementation that observes events from the file system is intended
 * to map directly on to the native file event notification facility where
 * available, or to use a primitive mechanism, such as polling, when a native
 * facility is not available. Consequently, many of the details on how events
 * are detected, their timeliness, and whether their ordering is preserved are
 * highly implementation specific. For example, when a file in a watched
 * directory is modified then it may result in a single {@link
 * StandardWatchEventKinds#ENTRY_MODIFY ENTRY_MODIFY} event in some
 * implementations but several events in other implementations. Short-lived
 * files (meaning files that are deleted very quickly after they are created)
 * may not be detected by primitive implementations that periodically poll the
 * file system to detect changes.