This event is a parallel dispatch event - you must use <code>event.enqueueWork</code> to interact with the game state in any way.
CommonSetup is fired after all the [[Events/RegistryEvent/1.16]]s, but before the [[Events/FMLClientSetupEvent/1.16]] and [[Events/FMLDedicatedServerSetupEvent/1.16]]. This makes it useful for non-specific mod setup.
Register it any way you would normally register an event.