Note I have a lot of mods installed on this Forge installation and thus there may be conflicts... I just noticed that RC is causing an abundance of errors when it comes to using a Pregenerator due to cascading world gen lag and that sort of thing, and now this error popped up after I somewhat resolved that issue.

Anyhow,

Description

I'm attempting to pregenerate a world for a modded server, and RC has been plopping out a menagerie of errors, and has (seemingly...?) given a crash, the cause of which appears to be some mob spawner for a boss in one of the generated structures. Pregeneration is done using this for this report.

Generic info

Running on Forge 1.12.2-14.23.5.2854 (latest Recommended),

-Xms10G -Xmx10G -XX:+UseG1GC -XX:+UnlockExperimentalVMOptions -XX:MaxGCPauseMillis=100 -XX:+DisableExplicitGC -XX:TargetSurvivorRatio=90 -XX:G1NewSizePercent=50 -XX:G1MaxNewSizePercent=80 -XX:G1MixedGCLiveThresholdPercent=50 -XX:+AlwaysPreTouch

start parameters for the .bat file used for launching the server.

Biomes o' Plenty is used for extra biomes/generation. This error does not appear when running the exact same mod setup with OTG generation instead.

The server crash log:
https://pastebin.com/HB1Te2rJ