I've recently been using the ability to fly under the world to edit blocks on bedrock level for many of my games. I was so excited for 15w37a when it came out as I was having a bug in 15w36d where I couldn't edit command blocks (my life). Now when I fall out of the world in super flat (no presets, on server, with levitation effect 251) My game crashes. Upon restarting and re-joining server, I get to fly under the world and fly back up to positive Y-levels. Until I fall out of the world again.

Crash report (Please note I am using Simplecraft V15 which was made for 1.7)

// Hi. I'm Minecraft, and I'm a crashaholic.

Time: 9/12/15 3:25 PM
Description: Unexpected error

java.lang.ArrayIndexOutOfBoundsException: -1
at aqz.a(SourceFile:552)
at bap.b(SourceFile:152)
at bap.a(SourceFile:70)
at bap.a(SourceFile:53)
at bah.a(SourceFile:217)
at bkp.a(SourceFile:1036)
at azx.as(SourceFile:954)
at azx.a(SourceFile:377)
at net.minecraft.client.main.Main.main(SourceFile:125)

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

– Head –
Stacktrace:
at aqz.a(SourceFile:552)
at bap.b(SourceFile:152)
at bap.a(SourceFile:70)
at bap.a(SourceFile:53)
at bah.a(SourceFile:217)

– Affected level –
Details:
Level name: MpServer
All players: 1 total; [bkc['GabeTheDog'/178, l='MpServer', x=-96498.60, y=-0.10, z=-964995.79]]
Chunk stats: MultiplayerChunkCache: 440, 440
Level seed: 0
Level generator: ID 01 - flat, ver 0. Features enabled: false
Level generator options:
Level spawn location: World: (187,13,24579), Chunk: (at 11,0,3 in 11,1536; contains blocks 176,0,24576 to 191,255,24591), Region: (0,48; contains chunks 0,1536 to 31,1567, blocks 0,0,24576 to 511,255,25087)
Level time: 92588039 game time, 1000 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: 1 total; [bkc['GabeTheDog'/178, l='MpServer', x=-96498.60, y=-0.10, z=-964995.79]]
Retry entities: 0 total; []
Server brand: vanilla
Server type: Non-integrated multiplayer server
Stacktrace:
at bid.a(SourceFile:366)
at azx.b(SourceFile:2369)
at azx.a(SourceFile:391)
at net.minecraft.client.main.Main.main(SourceFile:125)

– System Details –
Details:
Minecraft Version: 15w37a
Operating System: Mac OS X (x86_64) version 10.10.3
Java Version: 1.6.0_65, Apple Inc.
Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Apple Inc.
Memory: 123785072 bytes (118 MB) / 270774272 bytes (258 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: 0, tallocated: 0
Launched Version: 15w37a
LWJGL: 2.9.2
OpenGL: NVIDIA GeForce GT 640M OpenGL Engine GL version 2.1 NVIDIA-10.2.7 310.41.25f01, NVIDIA Corporation
GL Caps: Using GL 1.3 multitexturing.
Using GL 1.3 texture combiners.
Using framebuffer objects because ARB_framebuffer_object is supported and separate blending is supported.
Shaders are available because OpenGL 2.1 is supported.
VBOs are available because OpenGL 1.5 is supported.

Using VBOs: No
Is Modded: Probably not. Jar signature remains and client brand is untouched.
Type: Client (map_client.txt)
Resource Packs: SimpleCraft_V15 (incompatible)
Current Language: English (UK)
Profiler Position: N/A (disabled)
CPU: 4x Intel(R) Core(TM) i5-3335S CPU @ 2.70GHz