public class ExampleMod implements ModInitializer {
    [...]
    public static final ScreenHandlerType<BoxScreenHandler> BOX_SCREEN_HANDLER;
    [...]
    static {
        [...]
        //We use registerSimple here because our Entity is not an ExtendedScreenHandlerFactory
        //but a NamedScreenHandlerFactory.
        //In a later Tutorial you will see what ExtendedScreenHandlerFactory can do!
        BOX_SCREEN_HANDLER = ScreenHandlerRegistry.registerSimple(BOX, BoxScreenHandler::new);
    }

    @Override
    public void onInitialize() {

    }
}