crash on loading world (NullPointerException)

I got an error message after starting to load a world and minecraft breifly went to main menu (as it always does) but this time it didint go away, so I clicked to load the world again and it crashed with this error report:

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

Minecraft has stopped running because it encountered a problem; Exception in server tick loop
This error has been saved to /Users/rebekah/Library/Application Support/minecraft/crash-reports/crash-2012-11-01_12.05.58-server.txt for your convenience. Please include a copy of this file if you report this crash to anyone.



--- BEGIN ERROR REPORT 360efd8c --------
Generated 11/1/12 12:06 PM

- Minecraft Version: 1.4.2
- Operating System: Mac OS X (x86_64) version 10.8.2
- Java Version: 1.6.0_37, Apple Inc.
- Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
- Memory: 356197984 bytes (339 MB) / 581435392 bytes (554 MB) up to 1065025536 bytes (1015 MB)
- JVM Flags: 3 total; -Xbootclasspath/a:/System/Library/PrivateFrameworks/JavaApplicationLauncher.framework/Resources/LauncherSupport.jar -Xms512M -Xmx1024M
- AABB Pool Size: 2058 (115248 bytes; 0 MB) allocated, 2058 (115248 bytes; 0 MB) used
- Is Modded: Probably not
- Profiler Position: N/A (disabled)
- Vec3 Pool Size: 162 (9072 bytes; 0 MB) allocated, 162 (9072 bytes; 0 MB) used
- Player Count: 0 / 8; []
- World frequent villages 2;95 Entities: 0 total; []
- World frequent villages 2;95 Players: 0 total; []
- World frequent villages 2;95 Chunk Stats: ServerChunkCache: 123 Drop: 0
- Type: Integrated Server

java.lang.NullPointerException
	at bw.b(SourceFile:103)
	at bw.a(SourceFile:35)
	at im.b(SourceFile:457)
	at ge.a(SourceFile:73)
	at bce.b(SourceFile:72)
	at net.minecraft.server.MinecraftServer.r(SourceFile:731)
	at net.minecraft.server.MinecraftServer.q(SourceFile:661)
	at bcb.q(SourceFile:117)
	at net.minecraft.server.MinecraftServer.run(SourceFile:595)
	at fp.run(SourceFile:818)
--- END ERROR REPORT 79f6601d ----------