I haven't dug into this too much, but one example that causes it is:
/effect give @e[type=sheep] glowingIf there are sheep around, it gives out:
An unexpected error occured trying to execute that commandBut 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.NullPointerExceptionLinked issues
Comments 4
Please look at the list of affected versions before writing those comments. I created this report 32 minutes before you wrote that comment.
Also, green is a pretty bad colour choice for these comments, since it looks similar to an already added version in the mail.
I don't get any feedback in chat at all, however the exception is logged.
In one world the command started succeeding at one point (after setting the gamerule commandBlockOutput to false and true, though that is likely unrelated).
Confirmed for MC 1.13-pre5.