Greetings! Title says it all.
Happened while using Pixelmon and Metamorph together.
I am aware that mod compatibility isn't a main focus. Yet I hope that this can be fixed someday, somehow.

I hope the attached file provides enough information.
Happened while morphed into a Pixelmon and then threw potion of harming onto myself.
Also happens while walking (footstep sounds I suppose) and when being killed.
crash-2020-07-10_02.58.53-server.txt

First, this is a bug with Pixelmon and Changeling, not Pixelmon and Metamorph. As you can see in the crash log, the mod used is changeling-1.2_golem-1.12.2.jar (although it looks like I forgot to change the mod name of "MCLib core mod", oops!)

This is most likely caused by Pixelmon referencing client-side Minecraft code in a place they are not supposed to. It really should be fixed on Pixelmon's side, as it's likely a sign of a deeper issue, or another crash waiting to happen. And also, catching the exception would stop the footstep sounds from playing, which is no good!

Since you seem to be using Changeling, please report future bugs to the Changeling issue tracker instead.

The Pixelmon devs may be interested in this pull request made to the Animania mod, which demonstrates a similar problem and the solution taken: capnkirok/animaniamod#373

The Pixelmon Curseforge page is labeled with license "All Rights Reserved," and does not link to a source repository, so I am assuming their mod is closed source, so up to them I guess.

Oops, I forgot to switch back to Metamorph before recreating this error. My bad D:
Didn't consider yet that the error could be caused by Pixelmon. I will try your suggested solution.

Thanks for the quick reply and for your time and help!