Not entirely sure how this was caused, but I was in a creative world putting down a large selection of placeable blocks to create a test map for dynmap, and then the world crashed. Evidently there is an issue that causes TileEntityBattery.getCapability to recurse infinitely (see attached log).

crash-2020-05-13_02.18.11-client.txt

This is a duplicate of the already closed issues #253 and #287 , way back in november.
According to the crash log you are running version 1.12.2-3.3.0.63
Get the latest alpha version from curse: https://www.curseforge.com/minecraft/mc-mods/icbm-classic which is "ICBM - Classic 1.12.2-4.0.1b75-UNSTABLE build 75". It will be fixed in there and should be pretty stable.
Let me know how it goes.