Here is the crash report: ---- Minecraft Crash Report ----
// I just don't know what went wrong

Time: 12/19/12 6:02 PM
Description: Exception in world tick

java.lang.NullPointerException
at axz.a(SourceFile:640)
at ep.a(SourceFile:28)
at cg.b(SourceFile:349)
at axz.d(SourceFile:80)
at ayh.b(SourceFile:62)
at net.minecraft.client.Minecraft.l(SourceFile:1331)
at net.minecraft.client.Minecraft.J(SourceFile:582)
at net.minecraft.client.Minecraft.run(SourceFile:534)
at java.lang.Thread.run(Unknown Source)

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

– Head –
Stacktrace:
at axz.a(SourceFile:640)
at ep.a(SourceFile:28)
at cg.b(SourceFile:349)
at axz.d(SourceFile:80)
at ayh.b(SourceFile:62)

– Affected level –
Details:
Problem: Level is null!
Stacktrace:
at net.minecraft.client.Minecraft.l(SourceFile:1331)
at net.minecraft.client.Minecraft.J(SourceFile:582)
at net.minecraft.client.Minecraft.run(SourceFile:534)
at java.lang.Thread.run(Unknown Source)

– System Details –
Details:
Minecraft Version: 1.4.5
Operating System: Windows 7 (x86) version 6.1
Java Version: 1.7.0_09, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 458774272 bytes (437 MB) / 519110656 bytes (495 MB) up to 1037959168 bytes (989 MB)
JVM Flags: 2 total; -Xms512m -Xmx1024m
AABB Pool Size: 37333 (2090648 bytes; 1 MB) allocated, 21655 (1212680 bytes; 1 MB) used
Suspicious classes: No suspicious classes found.
IntCache: cache: 0, tcache: 0, allocated: 0, tallocated: 0
LWJGL: 2.4.2
OpenGL: Intel 945GM GL version 1.4.0 - Build 8.14.10.1930, Intel
Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Texture Pack: Default
Profiler Position: N/A (disabled)
Vec3 Pool Size: ~_ERROR~ NullPointerException: null