[MC-75236] Crash when fireball hits player with Thorns

Use the following steps to recreate:

1) Spawn dispenser, fire charge, redstone, redstone torch, and diamond armor with Thorns 3 (to increase chance of activation).

2) Point the dispenser at yourself, and make sure there is some distance between you and the dispenser, to let the fireball travel a bit.

3) Fill the dispenser with fireballs, put on your armor, and start activating the dispenser!

4) You most likely just crashed. Good job

This occurs because a Dispenser does not set the shooter for the Fireball, so when Thorns tries to damage the entity, it's null, resulting in the NullPointerException and crash.

Here is the crash log:

---- Minecraft Crash Report ----
// Who set us up the TNT?

Time: 12/9/14 11:26 PM
Description: Ticking entity

java.lang.NullPointerException: Ticking entity
at aps.b(SourceFile:50)
at aph.a(SourceFile:192)
at ape.a(SourceFile:121)
at ape.a(SourceFile:128)
at ape.a(SourceFile:202)
at wx.a(SourceFile:1992)
at ahm.a(SourceFile:35)
at ahi.t_(SourceFile:148)
at aqr.a(SourceFile:1407)
at qu.a(SourceFile:600)
at aqr.g(SourceFile:1385)
at aqr.i(SourceFile:1278)
at qu.i(SourceFile:478)
at net.minecraft.server.MinecraftServer.A(SourceFile:608)
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(Unknown Source)


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

-- Head --
Stacktrace:
at aps.b(SourceFile:50)
at aph.a(SourceFile:192)
at ape.a(SourceFile:121)
at ape.a(SourceFile:128)
at ape.a(SourceFile:202)
at wx.a(SourceFile:1992)
at ahm.a(SourceFile:35)
at ahi.t_(SourceFile:148)
at aqr.a(SourceFile:1407)
at qu.a(SourceFile:600)
at aqr.g(SourceFile:1385)

-- Entity being ticked --
Details:
Entity Type: SmallFireball (ahm)
Entity ID: 3281
Entity Name: Small Fireball
Entity's Exact location: -7.49, 71.33, -17.75
Entity's Block location: -8.00,71.00,-18.00 - World: (-8,71,-18), Chunk: (at 8,4,14 in -1,-2; contains blocks -16,0,-32 to -1,255,-17), Region: (-1,-1; contains chunks -32,-32 to -1,-1, blocks -512,0,-512 to -1,255,-1)
Entity's Momentum: 0.00, -0.03, 0.76
Entity's Rider: ~~ERROR~~ NullPointerException: null
Entity's Vehicle: ~~ERROR~~ NullPointerException: null
Stacktrace:
at aqr.i(SourceFile:1278)
at qu.i(SourceFile:478)

-- Affected level --
Details:
Level name: New World
All players: 1 total; [qx['zombiekiller753'/409, l='New World', x=-7.50, y=71.00, z=-17.06]]
Chunk stats: ServerChunkCache: 813 Drop: 0
Level seed: -8056990070904147019
Level generator: ID 00 - default, ver 1. Features enabled: true
Level generator options:
Level spawn location: 180.00,64.00,-84.00 - World: (180,64,-84), Chunk: (at 4,4,12 in 11,-6; contains blocks 176,0,-96 to 191,255,-81), Region: (0,-1; contains chunks 0,-32 to 31,-1, blocks 0,0,-512 to 511,255,-1)
Level time: 55446 game time, 5059 day time
Level dimension: 0
Level storage version: 0x04ABD - Anvil
Level weather: Rain time: 41715 (now: false), thunder time: 68079 (now: false)
Level game mode: Game mode: creative (ID 1). Hardcore: false. Cheats: true
Stacktrace:
at net.minecraft.server.MinecraftServer.A(SourceFile:608)
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(Unknown Source)

-- System Details --
Details:
Minecraft Version: 1.8.1
Operating System: Windows 8.1 (amd64) version 6.3
Java Version: 1.8.0_25, Oracle Corporation
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
Memory: 160050640 bytes (152 MB) / 513015808 bytes (489 MB) up to 1060372480 bytes (1011 MB)
JVM Flags: 6 total; -XX:HeapDumpPath=MojangTricksIntelDriversForPerformance_javaw.exe_minecraft.exe.heapdump -Xmx1G -XX:+UseConcMarkSweepGC -XX:+CMSIncrementalMode -XX:-UseAdaptiveSizePolicy -Xmn128M
IntCache: cache: 0, tcache: 0, allocated: 12, tallocated: 94
Profiler Position: N/A (disabled)
Player Count: 1 / 8; [qx['zombiekiller753'/409, l='New World', x=-7.50, y=71.00, z=-17.06]]
Type: Integrated Server (map_client.txt)
Is Modded: Probably not. Jar signature remains and both client + server brands are untouched.