Please add the following to the README:

To prevent crashing with Radium mod installed, Disable Radium's Hopper Optimizations mixin by adding the following line to the instance's minecraft/config/lithium.properties file:
mixin.block.hopper=false
(Edited to correct typo error)

My Apologies... I should have said "Radium" (not "Lithium")... Radium adds/uses the "Lithium.properties" config file.
(I will correct my OP accordingly).

I'm using the latest release of Radium (radium-mc1.20.1-0.12.2+git.5f80f74.jar)
Crashes unless the above entry is made to the lithium.properties file.

Link to actual crash : https://mclo.gs/PbMxsLK
Debug Log: https://mclo.gs/q62ts12

Ok, I tried Canary and the issue didn't happen. I've added it to README.

Thanks for the quick response. I'm sure others had this problem as well, and were thrown off by the Lithium reference when Radium is present instead. ;)

Please add the following to the README:

To prevent crashing with Lithium mod installed, Disable Lithium's Hopper optimizations by adding the following line to the instance's minecraft/config/lithium.properties file: mixin.block.hopper=false

Cannot reproduce. What version of Lithium are you using? Could you please upload the crash report and debug.log?