Thread: What does this mean? >.>

Code:

Exception in thread "Thread-0" java.lang.NullPointerException
        at net.com.codeusa.io.ActionSender.sendFriend(ActionSender.java:885)
        at net.com.codeusa.model.Player.destruct(Player.java:4534)
        at net.com.codeusa.Engine.removePlayer(Engine.java:314)
        at net.com.codeusa.Engine.connectAwaitingPlayers(Engine.java:296)
        at net.com.codeusa.Engine.run(Engine.java:166)
        at java.lang.Thread.run(Thread.java:619)

I am getting this frequently in my run.bat and its causing my server to crash. Whats it mean and how can I fix it?