How to get player data after use command

I writing a plugin that like parachute, when type the command it will TP on sky(y255) with slow falling potion added. But when i land to ground (around 10 sec to reach ground) i need to remove the potion effect asap, i tried while loop (stand != air) but my server freeze.
i googling alot of code but it only check player stand on block when use command only.

 

Thanks, it work well.

the result

Code:

 
@Override
public boolean onCommand(CommandSender sender, Command command, String label, String[] args) {
    if (sender instanceof Player) {
        Player player = (Player) sender;
        player.addPotionEffect(PotionEffectType.SLOW_FALLING.createEffect(900, 10));

        // to check is landing
        new BukkitRunnable() {
            @Override
            public void run() {
                if(player.getLocation().getBlock().getRelative(BlockFace.DOWN).getType() != Material.AIR) {
                    player.sendMessage(ChatColor.GREEN + "You reach the ground!");
                    if (player.hasPotionEffect(PotionEffectType.SLOW_FALLING)) {
                        player.removePotionEffect(PotionEffectType.SLOW_FALLING);
                    }
                    this.cancel();
                } else {
                    player.sendMessage(ChatColor.RED + "You are on mid-air!");
                }
            }
        }.runTaskTimer(Main.getPlugin(Main.class), 0, 20);
    }
    return false;
}