Crashed!?

hi guys i tried to log into mc to go on a LAN world with my cousin and after the logging in screen i got this
---- Minecraft Crash Report ----
// I just don't know what went wrong

Time: 01/08/12 14:05
Description: Exception in server tick loop

java.lang.NullPointerException
at ba.b(SourceFile:99)
at ba.a(SourceFile:31)
at gy.b(SourceFile:440)
at et.b(SourceFile:489)
at et.a(SourceFile:77)
at axx.b(SourceFile:72)
at net.minecraft.server.MinecraftServer.q(SourceFile:452)
at net.minecraft.server.MinecraftServer.p(SourceFile:383)
at axu.p(SourceFile:111)
at net.minecraft.server.MinecraftServer.run(SourceFile:316)
at ep.run(SourceFile:539)

Relevant Details:
- Minecraft Version: 1.3.1
- Operating System: Windows 7 (x86) version 6.1
- Java Version: 1.6.0_33, Sun Microsystems Inc.
- Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Sun Microsystems Inc.
- Memory: 298046360 bytes (284 MB) / 519110656 bytes (495 MB) up to 1037959168 bytes (989 MB)
- JVM Flags: 2 total; -Xms512m -Xmx1024m
- Type: Integrated Server
- Is Modded: Unknown (can't tell)
- Profiler Position: N/A (disabled)
- Player Count: 0 / 8; []
- World 1.3 Entities: 0 total; []
- World 1.3 Players: 0 total; []
- World 1.3 Chunk Stats: ServerChunkCache: 0 Drop: 0

i need help fast!