[MC-104592] Exception when stopping server

When stopping the server, sometimes this happens:

[11:30:02] [Server Shutdown Thread/INFO]: Stopping server
Exception in thread "Server Shutdown Thread" java.lang.NullPointerException
	at it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap$MapIterator.nextEntry(Long2ObjectOpenHashMap.java:511)
	at it.unimi.dsi.fastutil.longs.Long2ObjectOpenHashMap$ValueIterator.next(Long2ObjectOpenHashMap.java:706)
	at it.unimi.dsi.fastutil.objects.ObjectIterators.unwrap(ObjectIterators.java:228)
	at it.unimi.dsi.fastutil.objects.ObjectIterators.unwrap(ObjectIterators.java:242)
	at it.unimi.dsi.fastutil.objects.AbstractObjectCollection.toArray(AbstractObjectCollection.java:83)
	at java.util.ArrayList.<init>(ArrayList.java:177)
	at com.google.common.collect.Lists.newArrayList(Lists.java:129)
	at lr.a(SourceFile:159)
	at ls.a(SourceFile:884)
	at net.minecraft.server.MinecraftServer.a(SourceFile:372)
	at net.minecraft.server.MinecraftServer.u(SourceFile:397)
	at net.minecraft.server.MinecraftServer$2.run(SourceFile:738)

Code analysis by Marcono1234 can be found in this comment.