java.lang.NullPointerException
        at net.sf.l2j.gameserver.model.L2Attackable.removeKnownObject(L2Attackable.java:383)
        at net.sf.l2j.gameserver.model.L2MonsterInstance.removeKnownObject(L2MonsterInstance.java:139)
        at net.sf.l2j.gameserver.model.L2World.removeVisibleObject(L2World.java:470)
        at net.sf.l2j.gameserver.model.L2Object.decayMe(L2Object.java:474)
        at net.sf.l2j.gameserver.model.L2Character.onDecay(L2Character.java:787)
        at net.sf.l2j.gameserver.model.L2NpcInstance.onDecay(L2NpcInstance.java:1524)
        at net.sf.l2j.gameserver.model.L2Character$DecayTask.run(L2Character.java:404)
        at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
        at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
        at java.util.concurrent.FutureTask.run(Unknown Source)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$301(Unknown Source)
        at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
        at java.lang.Thread.run(Unknown Source)