Error on stopping server IPLOCK

10:27:42 [SEVERE] Error occurred while disabling IpLock v0.4 (Is it up to date?)

java.lang.NullPointerException
        at iplock.iplock.onDisable(iplock.java:98)
        at org.bukkit.plugin.java.JavaPlugin.setEnabled(JavaPlugin.java:217)
        at org.bukkit.plugin.java.JavaPluginLoader.disablePlugin(JavaPluginLoade
r.java:363)
        at org.bukkit.plugin.SimplePluginManager.disablePlugin(SimplePluginManag
er.java:400)
        at org.bukkit.plugin.SimplePluginManager.disablePlugins(SimplePluginMana
ger.java:393)
        at org.bukkit.craftbukkit.CraftServer.disablePlugins(CraftServer.java:25
1)
        at net.minecraft.server.MinecraftServer.stop(MinecraftServer.java:409)
        at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:489)
        at net.minecraft.server.ThreadServerApplication.run(SourceFile:492)