Cannot invoke "it.unimi.dsi.fastutil.objects.ObjectSet.remove(Object)" because "☃" is null

A new error since upgrading to 1.17:

[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 abk.b(SourceFile:202) ~[minecraft_server.jar:?]
at abe.a(SourceFile:906) ~[minecraft_server.jar:?]
at abp.a(SourceFile:449) ~[minecraft_server.jar:?]
at acj.a(SourceFile:944) ~[minecraft_server.jar:?]
at uf.a(SourceFile:114) ~[minecraft_server.jar:?]
at uf$a.a(SourceFile:42) ~[minecraft_server.jar:?]
at pm.a(SourceFile:21) ~[minecraft_server.jar:?]
at xm.run(SourceFile:18) ~[minecraft_server.jar:?]
at aqu.c(SourceFile:151) ~[minecraft_server.jar:?]
at aqy.c(SourceFile:23) ~[minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.b(SourceFile:783) ~[minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.c(SourceFile:167) ~[minecraft_server.jar:?]
at aqu.z(SourceFile:125) ~[minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.bg(SourceFile:765) ~[minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.z(SourceFile:759) ~[minecraft_server.jar:?]
at aqu.bq(SourceFile:110) ~[minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.y(SourceFile:743) ~[minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.x(SourceFile:696) ~[minecraft_server.jar:?]
at net.minecraft.server.MinecraftServer.a(SourceFile:274) ~[minecraft_server.jar:?]
at java.lang.Thread.run(Thread.java:831) [?:?]