[MC-132282] Some commands give an "unexpected error" despite succeeding

I haven't dug into this too much, but one example that causes it is:

/effect give @e[type=sheep] glowing

If there are sheep around, it gives out:

An unexpected error occured trying to execute that command

But the sheep do indeed glow.

It also happened with a few other commands, but I wasn't able to see a pattern yet.

The log contains this:

[21:45:41] [Client thread/FATAL]: Error executing task
java.util.concurrent.ExecutionException: java.lang.NullPointerException
	at java.util.concurrent.FutureTask.report(FutureTask.java:122) ~[?:1.8.0_171]
	at java.util.concurrent.FutureTask.get(FutureTask.java:192) ~[?:1.8.0_171]
	at k.a(SourceFile:175) [1.13-pre5.jar:?]
	at cep.c(SourceFile:769) [1.13-pre5.jar:?]
	at cep.a(SourceFile:373) [1.13-pre5.jar:?]
	at net.minecraft.client.main.Main.main(SourceFile:143) [1.13-pre5.jar:?]
Caused by: java.lang.NullPointerException