Ah! Wait! Are you running the wrong server jar? You need to run the forge jar file using a command prompt or terminal of sorts, not the minecraft server jar file.
You can register the block with your class in the registry event to override a vanilla block. Just remember to setRegistryName("minecraft", "lantern");
I was in a hurry when I wrote that and I noticed your setting/getting item refs and thought it could be an issue. I’m not sure what the issue is, can you please post your code on GitHub?