Hopper: java.lang.NullPointerException / Collision of entities

This is an automatically generated report based from the following crash: http://hopper.minecraft.net/crashes/minecraft/MCX-293266/

Crash:

java.lang.NullPointerException
	at bkv.a(SourceFile:303)
	at bkv.a(SourceFile:900)

The following is the first clean report made public about this crash:

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

Time: 2013/10/10 4:48 PM
Description: Unexpected error

java.lang.NullPointerException
	at bkv.a(SourceFile:303)
	at bkv.a(SourceFile:900)
	at bkv.b(SourceFile:814)
	at ays.ab(SourceFile:783)
	at ays.e(SourceFile:706)
	at net.minecraft.client.main.Main.main(SourceFile:103)


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

-- Head --
Stacktrace:
	at bkv.a(SourceFile:303)
	at bkv.a(SourceFile:900)

-- Affected level --
Details:
	Level name: MpServer
	All players: 1 total; [bip['0794918434bun'/268, l='MpServer', x=79.70, y=104.88, z=232.68]]
	Chunk stats: MultiplayerChunkCache: 225, 225
	Level seed: 0
	Level generator: ID 00 - default, ver 1. Features enabled: false
	Level generator options: 
	Level spawn location: World: (171,64,275), Chunk: (at 11,4,3 in 10,17; contains blocks 160,0,272 to 175,255,287), Region: (0,0; contains chunks 0,0 to 31,31, blocks 0,0,0 to 511,255,511)
	Level time: 227342 game time, 160692 day time
	Level dimension: 0
	Level storage version: 0x00000 - Unknown?
	Level weather: Rain time: 0 (now: false), thunder time: 0 (now: false)
	Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: false
	Forced entities: 93 total; [ww['Zombie'/1642, l='MpServer', x=98.50, y=112.00, z=262.50], wf['Creeper'/1375, l='MpServer', x=38.53, y=81.00, z=277.31], wr['Skeleton'/1634, l='MpServer', x=59.50, y=68.00, z=178.50], ww['Zombie'/1633, l='MpServer', x=130.50, y=63.00, z=221.50], ww['Zombie'/1638, l='MpServer', x=37.16, y=80.00, z=276.50], ww['Zombie'/1636, l='MpServer', x=14.50, y=91.00, z=246.50], uk['Bat'/1890, l='MpServer', x=37.53, y=37.84, z=168.47], wf['Creeper'/27, l='MpServer', x=6.56, y=36.00, z=157.97], uk['Bat'/1895, l='MpServer', x=116.24, y=20.82, z=288.47], wr['Skeleton'/34, l='MpServer', x=15.34, y=90.00, z=233.16], uu['Pig'/35, l='MpServer', x=11.50, y=97.00, z=286.66], wf['Creeper'/32, l='MpServer', x=8.09, y=68.50, z=177.66], wr['Skeleton'/38, l='MpServer', x=3.94, y=97.00, z=292.47], uu['Pig'/39, l='MpServer', x=3.94, y=98.00, z=289.03], un['Cow'/40, l='MpServer', x=-0.44, y=85.00, z=307.59], bip['0794918434bun'/268, l='MpServer', x=79.70, y=104.88, z=232.68], wr['Skeleton'/45, l='MpServer', x=19.50, y=72.00, z=171.50], wg['Enderman'/50, l='MpServer', x=26.63, y=97.00, z=226.78], wr['Skeleton'/48, l='MpServer', x=23.94, y=94.00, z=238.44], uu['Pig'/54, l='MpServer', x=27.50, y=80.00, z=291.50], uu['Pig'/53, l='MpServer', x=21.50, y=78.00, z=302.78], uu['Pig'/52, l='MpServer', x=20.03, y=78.00, z=298.03], wr['Skeleton'/1862, l='MpServer', x=75.14, y=24.00, z=306.50], wr['Skeleton'/1863, l='MpServer', x=84.47, y=25.00, z=310.52], wr['Skeleton'/1860, l='MpServer', x=81.50, y=25.00, z=309.50], wr['Skeleton'/1861, l='MpServer', x=80.50, y=25.00, z=309.50], uk['Bat'/1581, l='MpServer', x=34.66, y=52.26, z=235.31], uk['Bat'/1578, l='MpServer', x=103.50, y=12.46, z=180.31], uk['Bat'/1576, l='MpServer', x=57.00, y=60.51, z=226.50], uk['Bat'/1577, l='MpServer', x=68.31, y=59.89, z=235.94], uk['Bat'/67, l='MpServer', x=39.78, y=50.00, z=225.63], uu['Pig'/72, l='MpServer', x=38.16, y=87.00, z=291.03], uy['Squid'/895, l='MpServer', x=144.50, y=62.06, z=233.97], uk['Bat'/1588, l='MpServer', x=105.41, y=33.23, z=288.41], wg['Enderman'/94, l='MpServer', x=68.63, y=90.00, z=221.44], uk['Bat'/1587, l='MpServer', x=103.16, y=34.00, z=303.44], um['Chicken'/89, l='MpServer', x=60.28, y=89.00, z=166.66], um['Chicken'/102, l='MpServer', x=68.38, y=84.00, z=162.59], um['Chicken'/103, l='MpServer', x=69.75, y=92.00, z=167.53], uk['Bat'/1340, l='MpServer', x=101.25, y=36.32, z=308.13], wr['Skeleton'/854, l='MpServer', x=14.50, y=72.00, z=174.50], uk['Bat'/96, l='MpServer', x=55.34, y=60.10, z=229.63], uk['Bat'/111, l='MpServer', x=64.25, y=60.10, z=218.75], um['Chicken'/104, l='MpServer', x=69.50, y=90.00, z=166.50], ww['Zombie'/114, l='MpServer', x=80.72, y=99.00, z=262.34], wf['Creeper'/1800, l='MpServer', x=120.50, y=67.00, z=154.50], wf['Creeper'/113, l='MpServer', x=75.56, y=109.00, z=270.88], ww['Zombie'/1797, l='MpServer', x=18.50, y=15.00, z=186.50], wf['Creeper'/1558, l='MpServer', x=128.50, y=66.00, z=176.50], wf['Creeper'/1553, l='MpServer', x=50.50, y=67.00, z=164.50], wf['Creeper'/1763, l='MpServer', x=103.73, y=34.23, z=312.55], ww['Zombie'/136, l='MpServer', x=95.50, y=54.00, z=238.66], ww['Zombie'/139, l='MpServer', x=88.41, y=99.00, z=266.06], ww['Zombie'/138, l='MpServer', x=94.47, y=103.00, z=239.97], ww['Zombie'/140, l='MpServer', x=88.56, y=101.08, z=267.72], uu['Pig'/143, l='MpServer', x=85.44, y=87.00, z=284.91], wf['Creeper'/1764, l='MpServer', x=121.44, y=88.00, z=279.97], wf['Creeper'/945, l='MpServer', x=21.09, y=67.00, z=181.31], wr['Skeleton'/132, l='MpServer', x=91.50, y=66.00, z=170.50], ww['Zombie'/946, l='MpServer', x=23.50, y=83.00, z=264.50], wf['Creeper'/1237, l='MpServer', x=7.34, y=85.00, z=250.00], wf['Creeper'/943, l='MpServer', x=12.50, y=68.00, z=188.50], uk['Bat'/1474, l='MpServer', x=82.11, y=58.09, z=311.05], wr['Skeleton'/1234, l='MpServer', x=33.50, y=50.00, z=168.50], wf['Creeper'/1235, l='MpServer', x=36.50, y=50.00, z=166.50], wf['Creeper'/1524, l='MpServer', x=29.59, y=84.00, z=259.00], qz['entity.ItemFrame.name'/175, l='MpServer', x=124.06, y=87.50, z=283.50], qz['entity.ItemFrame.name'/174, l='MpServer', x=124.06, y=87.50, z=285.50], wf['Creeper'/1735, l='MpServer', x=96.50, y=46.00, z=172.50], wr['Skeleton'/1267, l='MpServer', x=55.50, y=85.00, z=310.50], wr['Skeleton'/188, l='MpServer', x=128.50, y=65.00, z=180.50], ww['Zombie'/1265, l='MpServer', x=94.50, y=44.00, z=217.50], qz['entity.ItemFrame.name'/189, l='MpServer', x=137.50, y=87.50, z=286.94], qz['entity.ItemFrame.name'/178, l='MpServer', x=124.06, y=88.50, z=284.50], uu['Pig'/179, l='MpServer', x=116.47, y=80.00, z=284.53], uy['Squid'/901, l='MpServer', x=87.66, y=60.28, z=184.16], qz['entity.ItemFrame.name'/176, l='MpServer', x=124.06, y=87.50, z=281.50], uy['Squid'/900, l='MpServer', x=84.13, y=62.31, z=181.44], qz['entity.ItemFrame.name'/177, l='MpServer', x=124.06, y=88.50, z=282.50], ww['Zombie'/1757, l='MpServer', x=58.50, y=21.00, z=222.50], uy['Squid'/898, l='MpServer', x=86.56, y=60.66, z=182.94], uy['Squid'/897, l='MpServer', x=135.50, y=62.38, z=232.38], uu['Pig'/180, l='MpServer', x=116.53, y=80.00, z=284.47], uu['Pig'/181, l='MpServer', x=115.88, y=82.00, z=287.56], wr['Skeleton'/1434, l='MpServer', x=109.50, y=65.00, z=165.50], wf['Creeper'/196, l='MpServer', x=146.34, y=58.00, z=265.44], wf['Creeper'/1163, l='MpServer', x=97.50, y=68.00, z=282.50], wr['Skeleton'/194, l='MpServer', x=151.50, y=48.00, z=269.50], uk['Bat'/1412, l='MpServer', x=91.50, y=38.07, z=306.72], xm['Fireball'/1923, l='MpServer', x=79.50, y=103.00, z=232.50], wr['Skeleton'/972, l='MpServer', x=63.50, y=34.00, z=272.50], qz['entity.ItemFrame.name'/246, l='MpServer', x=180.50, y=103.50, z=291.06], qz['entity.ItemFrame.name'/247, l='MpServer', x=182.50, y=103.50, z=291.06]]
	Retry entities: 0 total; []
	Server brand: vanilla
	Server type: Integrated singleplayer server
Stacktrace:
	at bik.a(SourceFile:302)
	at ays.b(SourceFile:1924)
	at ays.e(SourceFile:720)
	at net.minecraft.client.main.Main.main(SourceFile:103)

-- System Details --
Details:
	Minecraft Version: 13w41a
	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, sharing), Oracle Corporation
	Memory: 36191496 bytes (34 MB) / 207597568 bytes (197 MB) up to 518979584 bytes (494 MB)
	JVM Flags: 2 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx512M
	AABB Pool Size: 1090 (61040 bytes; 0 MB) allocated, 1075 (60200 bytes; 0 MB) used
	IntCache: cache: 0, tcache: 0, allocated: 13, tallocated: 95
	Launched Version: 13w41a
	LWJGL: 2.9.0
	OpenGL: GeForce GTX 550 Ti/PCIe/SSE2 GL version 4.3.0, NVIDIA Corporation
	Is Modded: Probably not. Jar signature remains and client brand is untouched.
	Type: Client (map_client.txt)
	Resource Packs: []
	Current Language: English (US)
	Profiler Position: N/A (disabled)
	Vec3 Pool Size: 868 (48608 bytes; 0 MB) allocated, 604 (33824 bytes; 0 MB) used

You can see this full report at http://hopper.minecraft.net/crashes/minecraft/MCX-293266/16534915/