Multiplayer server crash at end of raids

When raid wave is nearly complete, final mob to find crashes the server.
Recreated twice, after two separate raids in the same area/village.

---- Minecraft Crash Report ----
// Surprise! Haha. Well, this is awkward.

Time: 12/8/18 3:36 PM
Description: Exception ticking world

java.util.ConcurrentModificationException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1442)
	at java.util.HashMap$KeyIterator.next(HashMap.java:1466)
	at ui.b(SourceFile:105)
	at arm.j(SourceFile:230)
	at arm.l(SourceFile:327)
	at aro.a(SourceFile:80)
	at uk.a(SourceFile:205)
	at net.minecraft.server.MinecraftServer.b(SourceFile:779)
	at to.b(SourceFile:337)
	at net.minecraft.server.MinecraftServer.a(SourceFile:716)
	at net.minecraft.server.MinecraftServer.run(SourceFile:582)
	at java.lang.Thread.run(Thread.java:748)


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

-- Head --
Thread: Server thread
Stacktrace:
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1442)
	at java.util.HashMap$KeyIterator.next(HashMap.java:1466)
	at ui.b(SourceFile:105)
	at arm.j(SourceFile:230)
	at arm.l(SourceFile:327)
	at aro.a(SourceFile:80)
	at uk.a(SourceFile:205)

-- Affected level --
Details:
	Level name: world
	All players: 2 total; [um['Momibelle'/78, l='world', x=-4440.27, y=66.00, z=-1039.94], um['gameboySAM'/597, l='world', x=-4444.06, y=66.00, z=-1036.17]]
	Chunk stats: ServerChunkCache: 3546
	Level seed: 8885681254330917499
	Level generator: ID 00 - default, ver 1. Features enabled: true
	Level generator options: {}
	Level spawn location: World: (-192,65,-144), Chunk: (at 0,4,0 in -12,-9; contains blocks -192,0,-144 to -177,255,-129), Region: (-1,-1; contains chunks -32,-32 to -1,-1, blocks -512,0,-512 to -1,255,-1)
	Level time: 76738824 game time, 34119699 day time
	Level dimension: 0
	Level storage version: 0x04ABD - Anvil
	Level weather: Rain time: 1 (now: false), thunder time: 1 (now: false)
	Level game mode: Game mode: survival (ID 0). Hardcore: false. Cheats: true
Stacktrace:
	at net.minecraft.server.MinecraftServer.b(SourceFile:779)
	at to.b(SourceFile:337)
	at net.minecraft.server.MinecraftServer.a(SourceFile:716)
	at net.minecraft.server.MinecraftServer.run(SourceFile:582)
	at java.lang.Thread.run(Thread.java:748)

-- System Details --
Details:
	Minecraft Version: 18w49a
	Operating System: Linux (amd64) version 4.4.0-96-generic
	Java Version: 1.8.0_191, Oracle Corporation
	Java VM Version: Java HotSpot(TM) 64-Bit Server VM (mixed mode), Oracle Corporation
	Memory: 4319215312 bytes (4119 MB) / 6372196352 bytes (6077 MB) up to 6372196352 bytes (6077 MB)
	JVM Flags: 2 total; -Xmx6144M -Xms6144M
	Player Count: 2 / 120; [um['Momibelle'/78, l='world', x=-4440.27, y=66.00, z=-1039.94], um['gameboySAM'/597, l='world', x=-4444.06, y=66.00, z=-1036.17]]
	Data Packs: vanilla
	Is Modded: Unknown (can't tell)
	Type: Dedicated Server (map_server.txt)