java.lang.NullPointerException
        at net.sf.l2j.gameserver.model.L2Attackable.removeKnownObject(L2Attackab
le.java:164)
        at net.sf.l2j.gameserver.model.L2MonsterInstance.removeKnownObject(L2Mon
sterInstance.java:78)
        at net.sf.l2j.gameserver.model.L2World.removeVisibleObject(L2World.java:
455)
        at net.sf.l2j.gameserver.model.L2Object.decayMe(L2Object.java:418)
        at net.sf.l2j.gameserver.model.L2Character.onDecay(L2Character.java:629)

        at net.sf.l2j.gameserver.model.L2NpcInstance.onDecay(L2NpcInstance.java:
934)
        at net.sf.l2j.gameserver.model.L2Character$DecayTask.run(L2Character.jav
a:339)
        at java.util.concurrent.Executors$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)