Does not specify environment in FMJ

This means its loaded by servers and causes a crash. You should specify that the mod is client sided here: https://github.com/isXander/CullLessLeaves/blob/trunk/src/main/resources/fabric.mod.json#L16

tested with https://modrinth.com/mod/cull-less-leaves/version/1.4.2+1.21-fabric on mc 1.20.1