Not a crash but with Wither Skeleton Tweaks installed a blaze spawner spawns hundreds of wither skeletons, with your mod removed all is good.

Minecraft 1.12
Tried a few Forge versions and using 14.21.1.2400

This is technically the result of a config option. But, due to a very recent forge change, I think i can stop them spawning at a spawner like that.

There was around 500 plus within a few seconds, the image above is what I captured in peaceful mode which they spawn in then disappear.
I mean a wither skeleton spawner would be amazing but that was a little excess :)