Move Player To World Not Working ?!

Hello guys,
I am a Bukkit plugin maker beginner,
and I have a server and I wanna create just a simple plugin to move players.
(In future more things)
And I don't know what I am doing wrong!?

This is my error:

Code:

[22:10:02 INFO]: UMADSON issued server command: /skyblock
[22:10:02 ERROR]: null
org.bukkit.command.CommandException: Unhandled exception executing command 'skyblock' in plugin ServerPlugin v0.1
  at org.bukkit.command.PluginCommand.execute(PluginCommand.java:46) ~[minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:140) ~[minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  at org.bukkit.craftbukkit.v1_10_R1.CraftServer.dispatchCommand(CraftServer.java:625) ~[minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  at net.minecraft.server.v1_10_R1.PlayerConnection.handleCommand(PlayerConnection.java:1296) [minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  at net.minecraft.server.v1_10_R1.PlayerConnection.a(PlayerConnection.java:1156) [minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  at net.minecraft.server.v1_10_R1.PacketPlayInChat.a(SourceFile:37) [minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  at net.minecraft.server.v1_10_R1.PacketPlayInChat.a(SourceFile:9) [minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  at net.minecraft.server.v1_10_R1.PlayerConnectionUtils$1.run(SourceFile:13) [minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source) [?:1.8.0_92]
  at java.util.concurrent.FutureTask.run(Unknown Source) [?:1.8.0_92]
  at net.minecraft.server.v1_10_R1.SystemUtils.a(SourceFile:45) [minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  at net.minecraft.server.v1_10_R1.MinecraftServer.D(MinecraftServer.java:690) [minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  at net.minecraft.server.v1_10_R1.DedicatedServer.D(DedicatedServer.java:360) [minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  at net.minecraft.server.v1_10_R1.MinecraftServer.C(MinecraftServer.java:646) [minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  at net.minecraft.server.v1_10_R1.MinecraftServer.run(MinecraftServer.java:550) [minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  at java.lang.Thread.run(Unknown Source) [?:1.8.0_92]
Caused by: java.lang.NullPointerException
  at org.bukkit.craftbukkit.v1_10_R1.entity.CraftPlayer.teleport(CraftPlayer.java:460) ~[minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  at org.bukkit.craftbukkit.v1_10_R1.entity.CraftEntity.teleport(CraftEntity.java:226) ~[minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  at net.woosh.hub.main.Main.onCommand(Main.java:64) ~[?:?]
  at org.bukkit.command.PluginCommand.execute(PluginCommand.java:44) ~[minecraft_server.1.9.jar:git-Bukkit-10c10b3]
  ... 15 more

And this is My Main File:

Code:

package net.woosh.hub.main;

import java.util.logging.Logger;

import org.bukkit.Bukkit;
import org.bukkit.ChatColor;
import org.bukkit.Location;
import org.bukkit.World;
import org.bukkit.command.Command;
import org.bukkit.command.CommandSender;
import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.player.PlayerJoinEvent;
import org.bukkit.plugin.java.JavaPlugin;

import net.milkbowl.vault.economy.Economy;

public class Main extends JavaPlugin implements Listener {
  
   public static final String Prefix = ChatColor.DARK_GRAY+ "[" + ChatColor.DARK_PURPLE + ChatColor.BOLD + "Woosh" + ChatColor.RESET + ChatColor.DARK_GRAY + "] " + ChatColor.DARK_PURPLE;
  public static Economy econ = null;
  

   public void onEnable(){    
     Bukkit.getServer().getPluginManager().registerEvents(this, this);
   }
  
  
   @Override
   public void onDisable(){
    
   }
  
   @EventHandler
   public void onPlayerJoin(PlayerJoinEvent e){
     Player user = e.getPlayer();
     String message = Prefix + "Welcome, " + ChatColor.GOLD + user.getName() + ChatColor.DARK_PURPLE + "!";
     user.sendMessage(message);
   }
  
  @SuppressWarnings("unused")
  public int onlinePlayers() {
  int a = 0;
  
  for (Player player : getServer().getOnlinePlayers()) {
  a++;
  }
  
  return a;
  }
  Logger log = Logger.getLogger("minecraft");
  
  @Override
  public boolean onCommand(CommandSender sender, Command cmd, String commandLabel, String[] args){
     if(!(sender instanceof Player)){
       sender.sendMessage(Prefix + "You aren't a player!");
       return false;
     }
  if(cmd.getName().equalsIgnoreCase("skyblock") && args.length == 0){
  Player player = (Player) sender;
  World p = Bukkit.getWorld("skyblock");
  player.teleport(new Location(p, -232, 51, -8.5));
  player.sendMessage(Prefix + "You're now in the Skyblock!");
  return true;
  }else if(args.length == 1){
     Player player = (Player) sender;
     String vip = args[1];
    
     if(vip.equalsIgnoreCase("VIP")){
    
     for (Player all : Bukkit.getOnlinePlayers()) {
  if (all.hasPermission("woosh.perm")){
  World p = Bukkit.getWorld("skyblock");
  player.teleport(new Location(p, -232, 51, -8.5));
  player.sendMessage(Prefix + "You're now in the Skyblock!");
  }else{
  player.sendMessage(ChatColor.RED + "You don't have permissions to use this command!");
  }
     }
     }else{
       player.sendMessage(Prefix + "Unknow argument!");
     }
  }else if(args.length > 1){
     Player player = (Player) sender;
     player.sendMessage(Prefix + "Too much arguments!");
     }
  return false;
   }
}

And here is my plugin.yml file:

Code:

name: ServerPlugin
main: net.woosh.hub.main.Main
version: 0.1
author: UMADSON
prefix: Woosh
commands:
  skyblock:
  description: Goto VIP Skyblock!
  usage: /<command>

I hope that you know!
Sorry for bad English
Sorry for long question!