watchdog server kill froze server because of exception

Stack trace

[17:42:30] [Paper Watchdog Thread/INFO]: Stopping main thread (Ignore any thread death message you see! - DO NOT REPORT THREAD DEATH TO PAPER)
[17:42:30] [Paper Watchdog Thread/INFO]: Stopping server
[17:42:30] [Paper Watchdog Thread/INFO]: Saving players
[17:42:30] [Paper Watchdog Thread/WARN]: Asynchronous Save Players! Blocking thread until it returns 
java.lang.IllegalStateException: null
	at io.papermc.paper.util.MCUtil.ensureMain(MCUtil.java:350) [?:1.21-DEV-0e02aa5]
	at net.minecraft.server.players.PlayerList.h(PlayerList.java:1185) [?:1.21-DEV-0e02aa5]
	at net.minecraft.server.MinecraftServer.v(MinecraftServer.java:1060) [?:1.21-DEV-0e02aa5]
	at net.minecraft.server.dedicated.DedicatedServer.v(DedicatedServer.java:845) [?:1.21-DEV-0e02aa5]
	at net.minecraft.server.MinecraftServer.close(MinecraftServer.java:1006) [?:1.21-DEV-0e02aa5]
	at org.spigotmc.WatchdogThread.run(WatchdogThread.java:231) [?:1.21-DEV-0e02aa5]

Plugin and Datapack List

not relevant

Actions to reproduce (if known)

Sleep in the main thread for over 10 seconds then paper kills the server and the exception occurs

Paper version

[13:17:25 INFO]: Checking version, please wait...
[13:17:25 INFO]: This server is running Paper version 1.21-DEV-master@2288e13 (2024-07-13T21:34:15Z) (Implementing API version 1.21-R0.1-SNAPSHOT)
You are running the latest version
Previous version: 1.21-DEV-5a503d7 (MC: 1.21)
We use the reobf version so its selfbuild, but this is 1.21-63

Other

this error occurs because the PaperWatchdogThread (which kills / stop the server) is not the main tick thread

Start with the stack trace and the reported reproduction: sleep on the main thread for over 10 seconds. Read org/spigotmc/WatchdogThread.java, io/papermc/paper/util/MCUtil.java, and the referenced MinecraftServer and PlayerList methods to understand the shutdown path. Done means the watchdog-triggered shutdown no longer freezes or raises this main-thread exception.