[18w21a] Exception ticking world

Not sure how to reproduce yet. Occurred on an updated world from 18w20c (itself updated from 18w16a)

Had recently used /spreadplayers command

 Console displays this

[02:32:19] [Server thread/ERROR]: Encountered an unexpected exception
h: Exception ticking world
        at net.minecraft.server.MinecraftServer.w(SourceFile:698) ~[minecraft_se
rver.18w21a.jar:?]
        at rv.w(SourceFile:365) ~[minecraft_server.18w21a.jar:?]
        at net.minecraft.server.MinecraftServer.v(SourceFile:627) ~[minecraft_se
rver.18w21a.jar:?]
        at net.minecraft.server.MinecraftServer.run(SourceFile:532) [minecraft_s
erver.18w21a.jar:?]
        at java.lang.Thread.run(Unknown Source) [?:1.8.0_171]
Caused by: java.lang.NullPointerException
        at bwi.a(SourceFile:186) ~[minecraft_server.18w21a.jar:?]
        at bwi.a(SourceFile:92) ~[minecraft_server.18w21a.jar:?]
        at bwm.d(SourceFile:60) ~[minecraft_server.18w21a.jar:?]
        at brt.a(SourceFile:298) ~[minecraft_server.18w21a.jar:?]
        at sj.a(SourceFile:307) ~[minecraft_server.18w21a.jar:?]
        at sk.a(SourceFile:240) ~[minecraft_server.18w21a.jar:?]
        at bbo.a(SourceFile:135) ~[minecraft_server.18w21a.jar:?]
        at sk.i_(SourceFile:199) ~[minecraft_server.18w21a.jar:?]
        at net.minecraft.server.MinecraftServer.w(SourceFile:694) ~[minecraft_se
rver.18w21a.jar:?]
        ... 4 more
[02:32:19] [Server thread/ERROR]: This crash report has been saved to: C:\Users\
Redacted\Desktop\MCSRV_DEV\.\crash-reports\crash-2018-05-24_02.32.19-server.txt
[02:32:19] [Server thread/INFO]: Stopping server
[02:32:19] [Server thread/INFO]: Saving players
[02:32:19] [Server thread/INFO]: YT_Veritas0923 lost connection: Server closed
[02:32:19] [Server thread/INFO]: YT_Veritas0923 left the game
[02:32:19] [Server thread/INFO]: Saving worlds
[02:32:19] [Server thread/INFO]: Saving chunks for level 'world'/overworld
[02:32:19] [Server thread/INFO]: [STDERR]: java.lang.NullPointerException
[02:32:19] [Server thread/INFO]: [STDERR]:      at chw.a(SourceFile:348)
[02:32:19] [Server thread/INFO]: [STDERR]:      at java.util.HashMap$Values.forE
ach(Unknown Source)
[02:32:19] [Server thread/INFO]: [STDERR]:      at chw.a(SourceFile:345)
[02:32:19] [Server thread/INFO]: [STDERR]:      at java.util.stream.ForEachOps$F
orEachOp$OfRef.accept(Unknown Source)
[02:32:19] [Server thread/INFO]: [STDERR]:      at java.util.stream.ReferencePip
eline$3$1.accept(Unknown Source)
[02:32:19] [Server thread/INFO]: [STDERR]:      at java.util.HashMap$ValueSplite
rator.forEachRemaining(Unknown Source)
[02:32:19] [Server thread/INFO]: [STDERR]:      at java.util.stream.AbstractPipe
line.copyInto(Unknown Source)
[02:32:19] [Server thread/INFO]: [STDERR]:      at java.util.stream.AbstractPipe
line.wrapAndCopyInto(Unknown Source)
[02:32:19] [Server thread/INFO]: [STDERR]:      at java.util.stream.ForEachOps$F
orEachOp.evaluateSequential(Unknown Source)
[02:32:19] [Server thread/INFO]: [STDERR]:      at java.util.stream.ForEachOps$F
orEachOp$OfRef.evaluateSequential(Unknown Source)
[02:32:19] [Server thread/INFO]: [STDERR]:      at java.util.stream.AbstractPipe
line.evaluate(Unknown Source)
[02:32:19] [Server thread/INFO]: [STDERR]:      at java.util.stream.ReferencePip
eline.forEach(Unknown Source)
[02:32:19] [Server thread/INFO]: [STDERR]:      at chw.i(SourceFile:344)
[02:32:19] [Server thread/INFO]: [STDERR]:      at chx.b(SourceFile:146)
[02:32:19] [Server thread/INFO]: [STDERR]:      at cff.a(SourceFile:101)
[02:32:19] [Server thread/INFO]: [STDERR]:      at cff.a(SourceFile:87)
[02:32:19] [Server thread/INFO]: [STDERR]:      at sk.a(SourceFile:781)
[02:32:19] [Server thread/INFO]: [STDERR]:      at sk.a(SourceFile:732)
[02:32:19] [Server thread/INFO]: [STDERR]:      at net.minecraft.server.Minecraf
tServer.a(SourceFile:446)
[02:32:19] [Server thread/INFO]: [STDERR]:      at net.minecraft.server.Minecraf
tServer.h_(SourceFile:471)
[02:32:19] [Server thread/INFO]: [STDERR]:      at net.minecraft.server.Minecraf
tServer.run(SourceFile:565)
[02:32:19] [Server thread/INFO]: [STDERR]:      at java.lang.Thread.run(Unknown
Source)
[02:32:19] [Server thread/INFO]: Saving chunks for level 'world'/the_nether
[02:32:19] [Server thread/INFO]: Saving chunks for level 'world'/the_end
[02:32:20] [Server Shutdown Thread/INFO]: Stopping server
[02:32:20] [Server Shutdown Thread/INFO]: Saving players
[02:32:20] [Server Shutdown Thread/INFO]: Saving worlds
[02:32:20] [Server Shutdown Thread/INFO]: Saving chunks for level 'world'/overwo
rld
[02:32:20] [Server Shutdown Thread/ERROR]: Caught previously unhandled exception
 :
[02:32:20] [Server Shutdown Thread/ERROR]: java.util.concurrent.RejectedExecutio
nException: Task java.util.concurrent.FutureTask@4fcbe7a7 rejected from java.uti
l.concurrent.ThreadPoolExecutor@410c102e[Terminated, pool size = 0, active threa
ds = 0, queued tasks = 0, completed tasks = 12179]

Crash Report

---- Minecraft Crash Report ----
 // Oops.
Time: 5/24/18 2:32 AM
 Description: Exception ticking world
java.lang.NullPointerException: Exception ticking world
 at bwi.a(SourceFile:186)
 at bwi.a(SourceFile:92)
 at bwm.d(SourceFile:60)
 at brt.a(SourceFile:298)
 at sj.a(SourceFile:307)
 at sk.a(SourceFile:240)
 at bbo.a(SourceFile:135)
 at sk.i_(SourceFile:199)
 at net.minecraft.server.MinecraftServer.w(SourceFile:694)
 at rv.w(SourceFile:365)
 at net.minecraft.server.MinecraftServer.v(SourceFile:627)
 at net.minecraft.server.MinecraftServer.run(SourceFile:532)
 at java.lang.Thread.run(Unknown Source)
A detailed walkthrough of the error, its code path and all known details is as follows:
 ---------------------------------------------------------------------------------------
– Head –
 Thread: Server thread
 Stacktrace:
 at bwi.a(SourceFile:186)
 at bwi.a(SourceFile:92)
 at bwm.d(SourceFile:60)
 at brt.a(SourceFile:298)
 at sj.a(SourceFile:307)
 at sk.a(SourceFile:240)
 at bbo.a(SourceFile:135)
 at sk.i_(SourceFile:199)
– Affected level –
 Details:
 Level name: world
 All players: 1 total; [sm['YT_Veritas0923'/273253, l='world', x=3662.70, y=81.17, z=49395.75]]
 Chunk stats: ServerChunkCache: 561 Drop: 0
 Level seed: 1505535
 Level generator: ID 00 - default, ver 1. Features enabled: true
 Level generator options: {}
 Level spawn location: World: (193,64,-8), Chunk: (at 1,4,8 in 12,-1; contains blocks 192,0,-16 to 207,255,-1), Region: (0,-1; contains chunks 0,-32 to 31,-1, blocks 0,0,-512 to 511,255,-1)
 Level time: 41549617 game time, 43758370 day time
 Level dimension: 0
 Level storage version: 0x04ABD - Anvil
 Level weather: Rain time: 13693 (now: false), thunder time: 91571 (now: false)
 Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: false
 Stacktrace:
 at net.minecraft.server.MinecraftServer.w(SourceFile:694)
 at rv.w(SourceFile:365)
 at net.minecraft.server.MinecraftServer.v(SourceFile:627)
 at net.minecraft.server.MinecraftServer.run(SourceFile:532)
 at java.lang.Thread.run(Unknown Source)
– System Details –
 Details:
 Minecraft Version: 18w21a
 Operating System: Windows 8.1 (amd64) version 6.3
 Java Version: 1.8.0_171, Oracle Corporation
 Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
 Memory: 420519624 bytes (401 MB) / 1249378304 bytes (1191 MB) up to 2854223872 bytes (2722 MB)
 JVM Flags: 0 total; 
 Profiler Position: N/A (disabled)
 Player Count: 1 / 100; [sm['YT_Veritas0923'/273253, l='world', x=3662.70, y=81.17, z=49395.75]]
 Data Packs: vanilla, file/veritas0923 (incompatible)
 Is Modded: Unknown (can't tell)
 Type: Dedicated Server (map_server.txt)