The bug
I received the error java.lang.NullPointerException: Cannot invoke "it.unimi.dsi.fastutil.objects.ObjectSet.remove(Object)" because "$$4" is null
in my game's log today. I am unsure of the cause of the issue, because I wasn't monitoring my game's log at the time.
As of 1.18 Pre-release 7, this causes a crash.
Exception in server tick loop
java.lang.NullPointerException: Cannot invoke "it.unimi.dsi.fastutil.objects.ObjectSet.remove(Object)" because "$$4" is null
at adb.b(SourceFile:230)
at acv.a(SourceFile:1044)
at adg.a(SourceFile:457)
at aea.a(SourceFile:927)
at vp.a(SourceFile:114)
at vp$a.a(SourceFile:42)
at qt.a(SourceFile:21)
at yx.run(SourceFile:18)
at auh.c(SourceFile:151)
at aul.c(SourceFile:23)
at net.minecraft.server.MinecraftServer.b(SourceFile:780)
at net.minecraft.server.MinecraftServer.c(SourceFile:162)
at auh.y(SourceFile:125)
at net.minecraft.server.MinecraftServer.bf(SourceFile:762)
at net.minecraft.server.MinecraftServer.y(SourceFile:756)
at auh.bp(SourceFile:110)
at net.minecraft.server.MinecraftServer.x(SourceFile:740)
at net.minecraft.server.MinecraftServer.w(SourceFile:688)
at net.minecraft.server.MinecraftServer.a(SourceFile:270)
at java.base/java.lang.Thread.run(Thread.java:833)
Linked issues
is duplicated by
Attachments
Comments

Reproduction steps in MC-231132

I experienced this in my creative test world, but it was Cannot invoke "it.unimi.dsi.fastutil.objects.ObjectSet.remove(Object)" because "☃" is null
, with a snowman icon instead of "☃".

Can confirm for 1.17, server hosted on CubedHost. Seems that it was triggered right after a player fell out of the world in The End dimension.
[29-Jul-2021 15:24:32] [INFO] <redacted_username> fell out of the world
[29-Jul-2021 15:24:33] [FATAL] Error executing task on Server
[29-Jul-2021 15:24:33] java.lang.NullPointerException: Cannot invoke "it.unimi.dsi.fastutil.objects.ObjectSet.remove(Object)" because "☃" is null
[29-Jul-2021 15:24:33] at abk.b(SourceFile:202) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at abe.a(SourceFile:906) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at abp.a(SourceFile:449) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at acj.a(SourceFile:944) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at uf.a(SourceFile:114) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at uf$a.a(SourceFile:42) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at pm.a(SourceFile:21) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at xm.run(SourceFile:18) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at aqu.c(SourceFile:151) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at aqy.c(SourceFile:23) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at net.minecraft.server.MinecraftServer.b(SourceFile:783) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at net.minecraft.server.MinecraftServer.c(SourceFile:167) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at aqu.z(SourceFile:125) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at net.minecraft.server.MinecraftServer.bg(SourceFile:765) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at net.minecraft.server.MinecraftServer.z(SourceFile:759) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at aqu.bq(SourceFile:110) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at net.minecraft.server.MinecraftServer.y(SourceFile:743) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at net.minecraft.server.MinecraftServer.x(SourceFile:696) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at net.minecraft.server.MinecraftServer.a(SourceFile:274) ~[minecraft-release-1.17.jar:?]
[29-Jul-2021 15:24:33] at java.lang.Thread.run(Thread.java:831) [?:?]

EDIT: A player entered the end and flew straight away
[15:15:50] [Server thread/INFO]: <PlayerName> fell out of the world
[15:15:51] [Server thread/FATAL]: Error executing task on Server
java.lang.NullPointerException: Cannot invoke "it.unimi.dsi.fastutil.objects.ObjectSet.remove(Object)" because "☃" is null
at net.minecraft.class_3204.method_14051(class_3204.java:202) ~[intermediary-server.jar:?]
at net.minecraft.class_3898.method_18713(class_3898.java:2218) ~[intermediary-server.jar:?]
at net.minecraft.class_3215.method_14096(class_3215.java:450) ~[intermediary-server.jar:?]
at net.minecraft.class_3244.method_12063(class_3244.java:927) ~[intermediary-server.jar:?]
at net.minecraft.class_2828.method_12272(class_2828.java:114) ~[intermediary-server.jar:?]
at net.minecraft.class_2828$class_2829.method_11054(class_2828.java:42) ~[intermediary-server.jar:?]
at net.minecraft.class_2600.method_11072(class_2600.java:21) ~[intermediary-server.jar:?]
at net.minecraft.class_3738.run(class_3738.java:18) ~[intermediary-server.jar:?]
at net.minecraft.class_1255.method_18859(class_1255.java:151) ~[intermediary-server.jar:?]
at net.minecraft.class_4093.method_18859(class_4093.java:23) ~[intermediary-server.jar:?]
at net.minecraft.server.MinecraftServer.method_24306(MinecraftServer.java:788) ~[intermediary-server.jar:?]
at net.minecraft.server.MinecraftServer.method_18859(MinecraftServer.java:164) ~[intermediary-server.jar:?]
at net.minecraft.class_1255.method_16075(class_1255.java:125) ~[intermediary-server.jar:?]
at net.minecraft.server.MinecraftServer.method_20415(MinecraftServer.java:770) ~[intermediary-server.jar:?]
at net.minecraft.server.MinecraftServer.method_16075(MinecraftServer.java:764) ~[intermediary-server.jar:?]
at net.minecraft.class_1255.method_5383(class_1255.java:110) ~[intermediary-server.jar:?]
at net.minecraft.server.MinecraftServer.method_16208(MinecraftServer.java:748) ~[intermediary-server.jar:?]
at net.minecraft.server.MinecraftServer.method_29741(MinecraftServer.java:701) ~[intermediary-server.jar:?]
at net.minecraft.server.MinecraftServer.method_29739(MinecraftServer.java:270) ~[intermediary-server.jar:?]
at java.lang.Thread.run(Thread.java:831) [?:?]

I've experienced something in 1.17.1.
java.lang.NullPointerException: Cannot invoke "abr.o_()" because "Ô??" is null
Is this somehow related?

@unknown That is probably MC-230525.

There is an error in 21w37a that is probably the same thing as this:
Error executing task on Server
java.lang.NullPointerException: Cannot invoke "it.unimi.dsi.fastutil.objects.ObjectSet.remove(Object)" because "$$3" is null
at abp.b(SourceFile:202) ~[21w37a.jar:?]
at abj.a(SourceFile:982) ~[21w37a.jar:?]
at abu.a(SourceFile:444) ~[21w37a.jar:?]
at aco.a(SourceFile:927) ~[21w37a.jar:?]
at ui.a(SourceFile:114) ~[21w37a.jar:?]
at ui$a.a(SourceFile:42) ~[21w37a.jar:?]
at pn.a(SourceFile:21) ~[21w37a.jar:?]
at xp.run(SourceFile:18) ~[21w37a.jar:?]
at ash.c(SourceFile:151) ~[21w37a.jar:?]
at asl.c(SourceFile:23) ~[21w37a.jar:?]
at net.minecraft.server.MinecraftServer.b(SourceFile:790) ~[21w37a.jar:?]
at net.minecraft.server.MinecraftServer.c(SourceFile:168) ~[21w37a.jar:?]
at ash.z(SourceFile:125) ~[21w37a.jar:?]
at net.minecraft.server.MinecraftServer.bg(SourceFile:772) ~[21w37a.jar:?]
at net.minecraft.server.MinecraftServer.z(SourceFile:766) ~[21w37a.jar:?]
at ash.c(SourceFile:134) ~[21w37a.jar:?]
at net.minecraft.server.MinecraftServer.y(SourceFile:751) ~[21w37a.jar:?]
at net.minecraft.server.MinecraftServer.x(SourceFile:694) ~[21w37a.jar:?]
at net.minecraft.server.MinecraftServer.a(SourceFile:274) ~[21w37a.jar:?]
at java.lang.Thread.run(Thread.java:831) [?:?]

Can confirm in 21w39a.

Also in 21w40a.

Can this still be reproduced in 1.18-pre3?
I'm not sure about pre3, but it is still an issue in pre4.
java.lang.NullPointerException: Cannot invoke "it.unimi.dsi.fastutil.objects.ObjectSet.remove(Object)" because "$$4" is null
at adb.b(SourceFile:230)
at acv.a(SourceFile:1023)
at adg.a(SourceFile:457)
at aea.a(SourceFile:927)
at vp.a(SourceFile:114)
at vp$b.a(SourceFile:16)
at qt.a(SourceFile:21)
at yx.run(SourceFile:18)
at auh.c(SourceFile:151)
at aul.c(SourceFile:23)
at net.minecraft.server.MinecraftServer.b(SourceFile:780)
at net.minecraft.server.MinecraftServer.c(SourceFile:162)
at auh.y(SourceFile:125)
at net.minecraft.server.MinecraftServer.bf(SourceFile:762)
at net.minecraft.server.MinecraftServer.y(SourceFile:756)
at auh.bp(SourceFile:110)
at net.minecraft.server.MinecraftServer.x(SourceFile:740)
at net.minecraft.server.MinecraftServer.w(SourceFile:688)
at net.minecraft.server.MinecraftServer.a(SourceFile:270)
at java.base/java.lang.Thread.run(Thread.java:833)

I just had this crash on my server as well, the stack trace matches exactly. One of the players that was online right before the crash claims they died and then the server crashed; the other player was doing inventory management with a chest. I've attached a log for this report. Server is on 1.18-pre7.
I was able to reproduce this issue by flying into a single exit end portal block while using an elytra. I'm unsure if this is the specific cause of this issue.