[MC-155618] Exception ticking world / ConcurrentModificationException

We were in the middle of a raid and the server crashed. This is possibly related to ~~MC-155345~~, but I'm not sure anyone "left" the raid.

I've attached crash-2019-06-29_12.31.20-server.txt which was generated.

And here is the relevant section from latest.log:

[12:31:20] [Server thread/ERROR]: Encountered an unexpected exception
m: Exception ticking world
        at net.minecraft.server.MinecraftServer.b(SourceFile:828) ~[server.jar:?]
        at ui.b(SourceFile:343) ~[server.jar:?]
        at net.minecraft.server.MinecraftServer.a(SourceFile:763) ~[server.jar:?]
        at net.minecraft.server.MinecraftServer.run(SourceFile:631) [server.jar:?]
        at java.lang.Thread.run(Thread.java:748) [?:1.8.0_212]
Caused by: java.util.ConcurrentModificationException
        at java.util.HashMap$HashIterator.nextNode(HashMap.java:1445) ~[?:1.8.0_212]
        at java.util.HashMap$KeyIterator.next(HashMap.java:1469) ~[?:1.8.0_212]
        at java.util.Collections$UnmodifiableCollection$1.next(Collections.java:1042) ~[?:1.8.0_212]
        at axf.y(SourceFile:259) ~[server.jar:?]
        at axf.o(SourceFile:389) ~[server.jar:?]
        at axh.a(SourceFile:72) ~[server.jar:?]
        at vi.a(SourceFile:325) ~[server.jar:?]
        at net.minecraft.server.MinecraftServer.b(SourceFile:824) ~[server.jar:?]
        ... 4 more
[12:31:20] [Server thread/ERROR]: This crash report has been saved to: /usr/local/minecraft-server/./crash-reports/crash-2019-06-29_12.31.20-server.txt
[12:31:20] [Server thread/INFO]: Stopping server
[12:31:20] [Server thread/INFO]: Saving players

The JVM also created its own log of the crash as hs_err_pid20640.log, which I've attached.

It also created a rather large java.core file, but it's 12GB and I'm not sure how I'd provide that to you. The problem is very repeatable, it happens every time I start the server, log in, and do much of anything. There's a ravager running about and I did strike it with a sword, which perhaps triggered the end of the raid, triggering the bug? I'll keep testing but it takes a long time when it creates the core file.