follow-up patch for [HER-1764] H3 needs in-UI quit/shutdown option

* Flash.java
    avoid NPE when "flashdrop" cookie not set

engine/src/main/java/org/archive/crawler/restlet/Flash.java

@@ -79,6 +79,9 @@ public class Flash {
     public static List<Flash> getFlashes(Request request) {
         List<Flash> flashes = new LinkedList<Flash>();
         Series<Cookie> cookies = request.getCookies();
+        if (cookies.getFirstValue("flashdrop") == null) {
+            return flashes;
+        }
         for (String dropbox : cookies.getFirstValue("flashdrop").split(",")) {
             if(dropbox!=null) {
                 Flash flash = dropboxes.remove(Long.parseLong(dropbox));