Custom mobs make game crash when go to the nether portal

Steps to Reproduce:
1.Tame custom mobs and make sure it's not in sitting state.
2.Go to the nether portal. (start from overworld or nether)
3.You must enter the nether portal before custom mobs enter by itself.

Observed Results:
Game crash and sometimes you lost your tamed custom mobs that it's not in sitting state.
Sometimes it can nothing happened. I recommended to do this again about 2-3 times to trigger this bug.

Expected Results:
Nothing crash the game and custom mobs still exist when successful to enter the nether portal all time.

I can't attach my files because of the privacy.
Sorry for that.