My son was opening his inventory during a game when he got this. He says it lagged for quite a while before the crash.

Crash dump follows... Not sure what the "sister game" verbiage is about.

---- Minecraft Crash Report ----
// You should try our sister game, Minceraft!

Time: 1/3/15 12:29 PM
Description: Exception ticking world

java.lang.NullPointerException: Exception ticking world
at azi.a(SourceFile:124)
at azi.apply(SourceFile:121)
at beh.a(SourceFile:53)
at beh.apply(SourceFile:50)
at bei.a(SourceFile:67)
at bei.a(SourceFile:90)
at azh.f(SourceFile:49)
at azh.c(SourceFile:39)
at bff.a(SourceFile:551)
at aqr.a(SourceFile:290)
at bif.b(SourceFile:20)
at arv.a(SourceFile:261)
at arv.a(SourceFile:49)
at arj.a(SourceFile:389)
at asr.a(SourceFile:86)
at bgt.a(SourceFile:472)
at qt.a(SourceFile:174)
at bff.a(SourceFile:854)
at qt.c(SourceFile:104)
at qt.d(SourceFile:116)
at aqr.a(SourceFile:276)
at aqr.f(SourceFile:272)
at aqr.p(SourceFile:657)
at aqr.d(SourceFile:208)
at bav.b(SourceFile:64)
at ato.a(SourceFile:445)
at qu.h(SourceFile:383)
at qu.c(SourceFile:202)
at net.minecraft.server.MinecraftServer.A(SourceFile:600)
at net.minecraft.server.MinecraftServer.z(SourceFile:532)
at cyk.z(SourceFile:155)
at net.minecraft.server.MinecraftServer.run(SourceFile:448)
at java.lang.Thread.run(Thread.java:695)

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

– Head –
Stacktrace:
at azi.a(SourceFile:124)
at azi.apply(SourceFile:121)
at beh.a(SourceFile:53)
at beh.apply(SourceFile:50)
at bei.a(SourceFile:67)
at bei.a(SourceFile:90)
at azh.f(SourceFile:49)
at azh.c(SourceFile:39)
at bff.a(SourceFile:551)
at aqr.a(SourceFile:290)
at bif.b(SourceFile:20)
at arv.a(SourceFile:261)
at arv.a(SourceFile:49)
at arj.a(SourceFile:389)
at asr.a(SourceFile:86)
at bgt.a(SourceFile:472)
at qt.a(SourceFile:174)
at bff.a(SourceFile:854)
at qt.c(SourceFile:104)
at qt.d(SourceFile:116)
at aqr.a(SourceFile:276)
at aqr.f(SourceFile:272)
at aqr.p(SourceFile:657)
at aqr.d(SourceFile:208)
at bav.b(SourceFile:64)
at ato.a(SourceFile:445)
at qu.h(SourceFile:383)
at qu.c(SourceFile:202)

– Affected level –
Details:
Level name: Teck
All players: 1 total; [qx['A_Glasser_in_IA'/3678008, l='Teck', x=307.41, y=71.54, z=653.73]]
Chunk stats: ServerChunkCache: 1273 Drop: 0
Level seed: 6094816622330487461
Level generator: ID 02 - largeBiomes, ver 0. Features enabled: true
Level generator options:
Level spawn location: 180.00,64.00,252.00 - World: (180,64,252), Chunk: (at 4,4,12 in 11,15; contains blocks 176,0,240 to 191,255,255), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
Level time: 93798 game time, 93798 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 78407 (now: false), thunder time: 13625 (now: false)
Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: true
Stacktrace:
at net.minecraft.server.MinecraftServer.A(SourceFile:600)
at net.minecraft.server.MinecraftServer.z(SourceFile:532)
at cyk.z(SourceFile:155)
at net.minecraft.server.MinecraftServer.run(SourceFile:448)
at java.lang.Thread.run(Thread.java:695)

– System Details –
Details:
Minecraft Version: 1.8.1
Operating System: Mac OS X (x86_64) version 10.9.5
Java Version: 1.6.0_65, Apple Inc.
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
Memory: 218383064 bytes (208 MB) / 1060372480 bytes (1011 MB) up to 1060372480 bytes (1011 MB)
JVM Flags: 5 total; -Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 99
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [qx['A_Glasser_in_IA'/3678008, l='Teck', x=307.41, y=71.54, z=653.73]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.