Doesn't seem to be covered here. I hit a dark druid with a fire aspect sword, and when the fire ticks procced I got the following crash. It looks like the druid AI doesn't know what to do when it successfully hurts itself with fire damage? Not totally sure. It should be possible to reproduce by shooting one with a Flame arrow, or by lighting one up with flint and steel.

Singleplayer. Forge is 14.23.1.2562, Betweenlands version is 3.2.1-alpha.

crash-2018-01-06_04.29.37-server.txt