I get this error all the time in my console. Hundreds of times per day:
https://pastebin.com/MDQBnQbs
My guess is an incompatibility with EliteMobs

If not, either way, could you add support for it as well?

Hi. I had a lot of changes and bugfixes in V5.6.4 I (think) I fixed this bug too, but not 100% sure. please test V5.6.4 and let me know if this bug is fixed there.

Still getting similar errors with v5.6.4
https://pastebin.com/hM9jPA9q

Hmm...

What is cmd for you mobs? Has you set it to cmd: '' for any mobs? I would like to see your config.yml file?

and please test v5.6.5 if my assumption is correct v5.6.5 should solve your problem.

I think both bugs has been fixed in v5.6.7 (which will release in a short while)

Been trying the new versions as you roll them out but i don't know if new errors are popping up or its the same one. This is the latest error i got:
https://pastebin.com/1ecQeWbg
Also getting this huge spam:
https://pastebin.com/ayG9dnv3

Now on v5.6.9 again unsure if its the same error but here it is:
https://pastebin.com/J0hAUJcr

It looks like the same bug.... Could you please enable debug using "/mh debug" and show me the lines in the console log before this error appears ?

Sure here is one:
https://pastebin.com/feLZY0zR

Thanks. Please now show me the configuration section for the Enderman in config.yml

Enderman settings

enderman:
enabled: true
message: ''
money:
amount: '1.0'
chance: 0.12
commands:
- chance: '0.33'
cmd: null
head:
drophead: false
value: '0'
chance: 0.33
message: ''

https://pastebin.com/D8wvCgVF
For proper formatting ^

I now see that it is probably cmd: null which is making the trouble.... I thik it should be either

cmd: '' (2xping)

or

cmd: [] (2xbrackets)

I have to investigate this...

I think I found a fix now. Please test build 649

https://fractal.lindegaard.one:8181/job/MobHunting/

Did it work?