Running "/say Anything here" from console or a command block throws this error. Happens on 3555-Spigot-2183145-6da8266, didn't test on the latest version (currently 9 versions behind).
[20:35:32] Async Chat Thread - #3/ERROR: Task failed
java.util.concurrent.CompletionException: java.lang.NullPointerException: Cannot invoke "net.minecraft.server.level.EntityPlayer.getBukkitEntity()" because "<parameter1>" is null
        at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:315) ~[?:?]
        at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:320) [?:?]
        at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1770) [?:?]
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1136) [?:?]
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:635) [?:?]
        at java.lang.Thread.run(Thread.java:833) [?:?]
Caused by: java.lang.NullPointerException: Cannot invoke "net.minecraft.server.level.EntityPlayer.getBukkitEntity()" because "<parameter1>" is null
        at net.minecraft.server.MinecraftServer.lambda$42(MinecraftServer.java:2303) ~[spigot-1.19.1-R0.1-SNAPSHOT.jar:3555-Spigot-2183145-6da8266]
        at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1768) ~[?:?]
        ... 3 more