Add attribute null check

src/serverP2P/FileWatcher.java

@@ -124,8 +124,10 @@ public abstract class FileWatcher implements Runnable {
 	 */
 	public void setStop() {
 		stop = true;
+		if (thread != null) {
 			thread.interrupt();
 		}
+	}
 
 	/** Init sha512 map.
 	 */