Add attribute null check

src/serverP2P/FileWatcher.java

@@ -124,7 +124,9 @@ public abstract class FileWatcher implements Runnable {
 	 */
 	public void setStop() {
 		stop = true;
-		thread.interrupt();
+		if (thread != null) {
+			thread.interrupt();
+		}
 	}
 
 	/** Init sha512 map.