Game either crashes or sends me back to the title screen when I attempt to enter world.

Play game

Crashed or sent me to title screen.

---- Minecraft Crash Report ----
// Why is it breaking

Time: 11/24/12 10:32 AM
Description: Exception in server tick loop

java.lang.NullPointerException
at cf.b(SourceFile:103)
at cf.a(SourceFile:35)
at iv.b(SourceFile:437)
at gm.a(SourceFile:72)
at bdr.b(SourceFile:73)
at net.minecraft.server.MinecraftServer.r(SourceFile:769)
at net.minecraft.server.MinecraftServer.q(SourceFile:686)
at bdo.q(SourceFile:117)
at net.minecraft.server.MinecraftServer.run(SourceFile:620)
at fy.run(SourceFile:856)

A detailed walkthrough of the error, its code path and all known details is as follows:
---------------------------------------------------------------------------------------

– System Details –
Details:
Minecraft Version: 1.4.5
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: 154630624 bytes (147 MB) / 519110656 bytes (495 MB) up to 1037959168 bytes (989 MB)
JVM Flags: 2 total; -Xms512m -Xmx1024m
AABB Pool Size: 202032 (11313792 bytes; 10 MB) allocated, 202032 (11313792 bytes; 10 MB) used
Suspicious classes: No suspicious classes found.
IntCache: cache: 0, tcache: 0, allocated: 1, tallocated: 63
Profiler Position: N/A (disabled)
Vec3 Pool Size: 162620 (9106720 bytes; 8 MB) allocated, 162620 (9106720 bytes; 8 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.