crash

Put the summary of the bug you're having here

What I expected to happen was...:
I should have moved forward

What actually happened was...:
it crashed

Steps to Reproduce:
Don't know. I was just playing normally and it crashed. I restarted and played for a minute or so and it crashed again. crash report below..

---- Minecraft Crash Report ----
// This doesn't make any sense!

Time: 19/03/14 7:39 PM
Description: Exception ticking world

java.lang.NullPointerException: Exception ticking world
	at mr.d(SourceFile:204)
	at ms.b(SourceFile:119)
	at net.minecraft.server.MinecraftServer.v(SourceFile:504)
	at net.minecraft.server.MinecraftServer.u(SourceFile:443)
	at btq.u(SourceFile:117)
	at net.minecraft.server.MinecraftServer.run(SourceFile:359)
	at lh.run(SourceFile:618)


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

-- Head --
Stacktrace:
	at mr.d(SourceFile:204)
	at ms.b(SourceFile:119)

-- Affected level --
Details:
	Level name: Isles
	All players: 1 total; [mv['ScarlettBlonde'/662, l='Isles', x=129.97, y=31.00, z=438.97]]
	Chunk stats: ServerChunkCache: 741 Drop: 216
	Level seed: 8294898366627015104
	Level generator: ID 00 - default, ver 1. Features enabled: true
	Level generator options: 
	Level spawn location: World: (-40,64,256), Chunk: (at 8,4,0 in -3,16; contains blocks -48,0,256 to -33,255,271), Region: (-1,0; contains chunks -32,0 to -1,31, blocks -512,0,0 to -1,255,511)
	Level time: 1481506 game time, 2226601 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 148118 (now: false), thunder time: 129971 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
Stacktrace:
	at net.minecraft.server.MinecraftServer.v(SourceFile:504)
	at net.minecraft.server.MinecraftServer.u(SourceFile:443)
	at btq.u(SourceFile:117)
	at net.minecraft.server.MinecraftServer.run(SourceFile:359)
	at lh.run(SourceFile:618)

-- System Details --
Details:
	Minecraft Version: 1.7.5
	Operating System: Windows 7 (x86) version 6.1
	Java Version: 1.6.0_20, Sun Microsystems Inc.
	Java VM Version: Java HotSpot(TM) Client VM (mixed mode), Sun Microsystems Inc.
	Memory: 90492944 bytes (86 MB) / 303931392 bytes (289 MB) up to 518979584 bytes (494 MB)
	JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M
	AABB Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
	IntCache: cache: 0, tcache: 0, allocated: 12, tallocated: 94
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 0 (0 bytes; 0 MB) allocated, 0 (0 bytes; 0 MB) used
	Player Count: 1 / 8; [mv['ScarlettBlonde'/662, l='Isles', x=129.97, y=31.00, z=438.97]]
	Type: Integrated Server (map_client.txt)
	Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.