I can't open my worlds and even if it opens it crashes.

Minecraft Crash Report ----
// Sorry 😞

Time: 6/07/13 9:49 AM
Description: Sending packet

java.lang.NullPointerException
at cm.b(SourceFile:116)
at cm.a(SourceFile:41)
at jx.b(SourceFile:436)
at hk.b(SourceFile:644)
at hk.a(SourceFile:79)
at bkr.b(SourceFile:73)
at net.minecraft.server.MinecraftServer.t(SourceFile:488)
at net.minecraft.server.MinecraftServer.s(SourceFile:405)
at bko.s(SourceFile:124)
at net.minecraft.server.MinecraftServer.run(SourceFile:339)
at hf.run(SourceFile:582)

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

– Head –
Stacktrace:
at cm.b(SourceFile:116)
at cm.a(SourceFile:41)

– Packet being sent –
Details:
Packet ID: 4
Packet class: fw
Stacktrace:
at jx.b(SourceFile:436)
at hk.b(SourceFile:644)
at hk.a(SourceFile:79)
at bkr.b(SourceFile:73)
at net.minecraft.server.MinecraftServer.t(SourceFile:488)
at net.minecraft.server.MinecraftServer.s(SourceFile:405)
at bko.s(SourceFile:124)
at net.minecraft.server.MinecraftServer.run(SourceFile:339)
at hf.run(SourceFile:582)

– System Details –
Details:
Minecraft Version: 1.6.1
Operating System: Windows 7 (x86) version 6.1
Java Version: 1.7.0_07, Oracle Corporation
Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Oracle Corporation
Memory: 66093056 bytes (63 MB) / 393175040 bytes (374 MB) up to 1037959168 bytes (989 MB)
JVM Flags: 1 total; -Xmx1G
AABB Pool Size: 4806 (269136 bytes; 0 MB) allocated, 4806 (269136 bytes; 0 MB) used
Suspicious classes: No suspicious classes found.
IntCache: cache: 0, tcache: 3, allocated: 1, tallocated: 63
Profiler Position: N/A (disabled)
Vec3 Pool Size: 1430 (80080 bytes; 0 MB) allocated, 1430 (80080 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.