I attempted to load a world and got this error upon trying to load a second time after being sent back to the home screen:

Minecraft has crashed!
----------------------

Minecraft has stopped running because it encountered a problem; Exception in server tick loop

A full error report has been saved to C:\Users\Erik\AppData\Roaming\.minecraft\crash-reports\crash-2013-02-27_21.00.43-server.txt - Please include a copy of that file (Not this screen!) if you report this crash to anyone; without it, they will not be able to help fix the crash

— BEGIN ERROR REPORT bb3bd4a --------
Full report at:
C:\Users\Erik\AppData\Roaming\.minecraft\crash-reports\crash-2013-02-27_21.00.43-server.txt
Please show that file to Mojang, NOT just this screen!

Generated 2/27/13 9:00 PM

– System Details –
Details:
Minecraft Version: 13w09b
Operating System: Windows 7 (amd64) version 6.1
Java Version: 1.7.0_07, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 260380328 bytes (248 MB) / 648740864 bytes (618 MB) up to 954466304 bytes (910 MB)
JVM Flags: 2 total; -Xms512m -Xmx1024m
AABB Pool Size: 33838 (1894928 bytes; 1 MB) allocated, 33838 (1894928 bytes; 1 MB) used
Suspicious classes: No suspicious classes found.
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
Profiler Position: N/A (disabled)
Vec3 Pool Size: 4428 (247968 bytes; 0 MB) allocated, 4428 (247968 bytes; 0 MB) used
Player Count: 0 / 8; []
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.

java.lang.NullPointerException
at ch.b(SourceFile:107)
at ch.a(SourceFile:39)
at jg.b(SourceFile:427)
at gt.b(SourceFile:669)
at gt.a(SourceFile:79)
at bja.b(SourceFile:73)
at net.minecraft.server.MinecraftServer.r(SourceFile:480)
at net.minecraft.server.MinecraftServer.q(SourceFile:397)
at bix.q(SourceFile:122)
at net.minecraft.server.MinecraftServer.run(SourceFile:331)
at go.run(SourceFile:573)
— END ERROR REPORT 9c3d410 ----------