if(sender instanceof Player){
    Player player = (Player)sender;
    ItemStack potion = new ItemStack(Material.POTION);
    PotionMeta meta = (PotionMeta) potion.getItemMeta();
    PotionEffect effect = new PotionEffect(PotionEffectType.FIRE_RESISTANCE, 20 * 60, 1);
    meta.addCustomEffect(effect, true);
    meta.setMainEffect(PotionEffectType.FIRE_RESISTANCE);
    potion.setItemMeta(meta);
    player.getInventory().addItem(potion);
}