The way you add e.g. ASPEN_LEAVES to the Vanilla Composter Block is not safe and sometimes causes a crash on startup.
A suggestion would be to fix this by doing it in FMLCommonSetupEvent and not in FMLLoadCompleteEvent

I attached the crash log so that you can read the error for yourselves
crash-2021-01-22_10.55.13-fml.txt

Fixed in latest beta.