An internal error occurred while attempting to perform this command

Code:

package Classes;


import net.md_5.bungee.api.ChatColor;

import org.bukkit.GameMode;
import org.bukkit.command.Command;
import org.bukkit.command.CommandExecutor;
import org.bukkit.command.CommandSender;
import org.bukkit.entity.Player;

public class Commands implements CommandExecutor {
   
    private Main plugin;
   
    public Commands(Main plugin) {
        this.plugin = plugin;
    }
   
    public boolean onCommand(CommandSender sender, Command cmd, String label,String[] args) {
       
        Player p = (Player) sender;
        String mode = args[0];
       
        if(!sender.hasPermission("qplugin.gm")){
            sender.sendMessage(ChatColor.RED + "You don't have permission.");
            return false;
        }
       
        if(args.length == 0 ){
            return false;
        }
       
        if (args.length > 0){
            if(mode.equals("0")){
                sender.sendMessage(ChatColor.GOLD + "You got survival!");
                p.setGameMode(GameMode.SURVIVAL);
                return true;
            } else if(mode.equals("1")){
                sender.sendMessage(ChatColor.GOLD + "You got creative!");
                p.setGameMode(GameMode.CREATIVE);
                return true;
            }else if(mode.equals("2")){
                sender.sendMessage(ChatColor.GOLD + "You got adventure!");
                p.setGameMode(GameMode.ADVENTURE);
                return true;
            }else if(mode.equals("3")){
                sender.sendMessage(ChatColor.GOLD + "You got spectator!");
                p.setGameMode(GameMode.SPECTATOR);
                return true;
            }
        }
       
        return false;       
    }
}

When I write: /gm

I get: An internal error occurred while attempting to perform this command

help please

 

Thank you