[help]npc Class Not Found

NPC 31921 class not found: null
FourSepulchersManager: spawned 31921
NPC 31922 class not found: null
FourSepulchersManager: spawned 31922
NPC 31923 class not found: null
FourSepulchersManager: spawned 31923
NPC 31924 class not found: null
FourSepulchersManager: spawned 31924
Exception in thread "main" java.lang.NullPointerException
        at net.sf.l2j.gameserver.instancemanager.FourSepulchersManager.clean(Fou
rSepulchersManager.java:220)
        at net.sf.l2j.gameserver.instancemanager.FourSepulchersManager.timeSelec
tor(FourSepulchersManager.java:186)
        at net.sf.l2j.gameserver.instancemanager.FourSepulchersManager.init(Four
SepulchersManager.java:154)
        at net.sf.l2j.gameserver.GameServer.<init>(GameServer.java:389)
        at net.sf.l2j.gameserver.GameServer.main(GameServer.java:699)