Could not pass event PlayerLoginEvent to CommandBook

CMDBOOK-2190 - Reported by unusualbob

On player login a stacktrace appears in the console.

```
13:11:48 [SEVERE] Could not pass event PlayerLoginEvent to CommandBook v539-57d1a9e
org.bukkit.event.EventException
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:427)
at org.bukkit.plugin.RegisteredListener.callEvent(RegisteredListener.java:62)
at org.bukkit.plugin.SimplePluginManager.fireEvent(SimplePluginManager.java:477)
at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:462)
at net.minecraft.server.v1_4_6.PlayerList.attemptLogin(PlayerList.java:275)
at net.minecraft.server.v1_4_6.PendingConnection.d(PendingConnection.java:121)
at net.minecraft.server.v1_4_6.PendingConnection.c(PendingConnection.java:45)
at net.minecraft.server.v1_4_6.DedicatedServerConnectionThread.a(DedicatedServerConnectionThread.java:44)
at net.minecraft.server.v1_4_6.DedicatedServerConnection.b(SourceFile:29)
at net.minecraft.server.v1_4_6.MinecraftServer.r(MinecraftServer.java:598)
at net.minecraft.server.v1_4_6.DedicatedServer.r(DedicatedServer.java:224)
at net.minecraft.server.v1_4_6.MinecraftServer.q(MinecraftServer.java:494)
at net.minecraft.server.v1_4_6.MinecraftServer.run(MinecraftServer.java:427)
at net.minecraft.server.v1_4_6.ThreadServerApplication.run(SourceFile:849)
Caused by: java.lang.NullPointerException
at com.sk89q.commandbook.session.SessionComponent.onLogin(SessionComponent.java:325)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:601)
at org.bukkit.plugin.java.JavaPluginLoader$1.execute(JavaPluginLoader.java:425)
... 13 more
```

CraftBukkit version git-Bukkit-1.4.6-R0.3-7-g48dcb3e-b2591jnks (MC: 1.4.6) (Implementing API version 1.4.6-R0.4-SNAPSHOT)