This is because neo forge has changed the way you register an object which can be seen in the following pr

neoforged/NeoForge#257

It now means As RegistryObject no longer exist

Issue currently with update to latest neo forge version in gradle, fix found by @thedarkcolour

Replaced with an extension function for DeferredHolder.getValue, which allows using the by syntax with any of the DeferredHolders returned by the DeferredRegister. Must use the following import: import thedarkcolour.kotlinforforge.neoforge.forge.getValue