OLD working version

        //Silverfish
        if (AllowSilver == false) {
            if (SpawnReason.NATURAL != null)
                if (event.getCreatureType() == CreatureType.SILVERFISH)
                    event.setCancelled(true);
        }

        NEW broken version

        if (event.getCreatureType() == CreatureType.SILVERFISH) {
            if (event.getSpawnReason() == SpawnReason.NATURAL);
                if ((CONFIG.getBoolean("Allow Silverfish", true)) == false);
                    event.setCancelled(true);
        }