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)
Linked issues
duplicates 1
Attachments
Comments 2

I was able to create a new singleplayer world in 18w21a using the seed in your crash report, -7151426765772607981
Ran around a bit. Went for a swim. Punched a tree. Did not crash.
I will try making a world in 1.12.2 or even an earlier snapshot and upgrading it.
This happened to me on a brand new world, as I could not get a 1.12.2 world to load past 0% in the new snapshots. Uploading the crash log as well.