This is major because this NPE prevents the server from stopping and it has to be manually killed. I have not yet waited long enough before killing the process to see if a watchdog will cleanup the process or not.

The error occurs while plugins are shutting down, it may be plugin related but there is nothing in the stack trace to indicate that.

This error happens once or more

java.lang.IllegalStateException: AsyncPlayerPreLoginEvent cannot be triggered asynchronously from primary server thread.java.lang.IllegalStateException: AsyncPlayerPreLoginEvent cannot be triggered asynchronously from primary server thread. at org.bukkit.plugin.SimplePluginManager.callEvent(SimplePluginManager.java:568) ~[spigot-1.16.1.jar:git-Spigot-6f4ff1b-983305b] at net.minecraft.server.v1_16_R1.LoginListener$LoginHandler.fireEvents(LoginListener.java:270) ~[spigot-1.16.1.jar:git-Spigot-6f4ff1b-983305b] at net.minecraft.server.v1_16_R1.LoginListener$2.run(LoginListener.java:220) [spigot-1.16.1.jar:git-Spigot-6f4ff1b-983305b]

Then world / player save completes and the last log line is

[21:52:13] [Server thread/ERROR]: java.lang.NullPointerException