Bukkit

What steps will reproduce the problem?
1.When kill the monster

What is the expected output? What do you see instead?
Nothing output is expected

What version of the product are you using?
Plugin 1.2.3.166 and 1.2.4.169

Do you have an error log of what happened? (Portal: check cache/error.txt)
org.apache.commons.lang.UnhandledException: Plugin Statistics v1.2.3 generated an exception while executing task 63539
        at org.bukkit.craftbukkit.v1_8_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:56)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
        at java.lang.Thread.run(Thread.java:745)
Caused by: java.lang.NullPointerException
        at com.wolvencraft.yasp.session.OnlineSession.killedCreature(OnlineSession.java:232)
        at com.wolvencraft.yasp.listeners.handlers.DeathHandler$MonsterDeath.run(DeathHandler.java:170)
        at org.bukkit.craftbukkit.v1_8_R1.scheduler.CraftTask.run(CraftTask.java:53)
        at org.bukkit.craftbukkit.v1_8_R1.scheduler.CraftAsyncTask.run(CraftAsyncTask.java:53)
        ... 3 more

Please provide any additional information below. (Plugin/Portal Version, Server Version, PHP Version. MySQL Version)
Server version : CraftBukkit version git-Bukkit-e6f9fd4 (MC: 1.8) (Implementing API version 1.8-R0.1-SNAPSHOT)
PHP version : PHP 5.5.9-1ubuntu4.6 (cli)
MySQL version : Ver 14.14 Distrib 5.5.40, for debian-linux-gnu (x86_64) using readline 6.3

If im not mistaken this happens only sometimes? Have you found out when this happens (which monster you have to kill)?