I'm working on a command for a bukkit plugin that lets you transfer worlds. In the section about saving the player's inventory, I'm getting a runtime error. My question is: Why is the error happening, and how can I prevent it?

The plugin code

        public void savePlayerInv(Player p, World w){
        File playerInvConfigFile = new File(plugin.getDataFolder() + File.separator + "players" + File.separator + p.getName(), "inventory.yml");
        FileConfiguration pInv = YamlConfiguration.loadConfiguration(playerInvConfigFile);
        PlayerInventory inv = p.getInventory();
        int i = 0;

        for (ItemStack stack : inv.getContents()) {
            //increment integer
            i++;
            String startInventory = w.getName() + ".inv." + Integer.toString(i);

            //save inv
            pInv.set(startInventory + ".amount", stack.getAmount());
            pInv.set(startInventory + ".durability", Short.toString(stack.getDurability()));
            pInv.set(startInventory + ".type", stack.getTypeId());
            //pInv.set(startInventory + ".enchantment", stack.getEnchantments());
            //TODO add enchant saveing
        }

        i = 0;
        for (ItemStack armor : inv.getArmorContents()){
                i++;
                String startArmor = w.getName() + ".armor." + Integer.toString(i);

                //save armor
                pInv.set(startArmor + ".amount", armor.getAmount());
                pInv.set(startArmor + ".durability", armor.getDurability());
                pInv.set(startArmor + ".type", armor.getTypeId());
                //pInv.set(startArmor + ".enchantment", armor.getEnchantments());
        }

        //save exp
        if (p.getExp() != 0) {
                pInv.set(w.getName() + ".exp", p.getExp());
        }
    }

The offending line

The stack trace complains about line 130, which is this line.

pInv.set(startInventory + ".amount", stack.getAmount());

The stack trace

2012-03-21 13:23:25 [SEVERE] null
org.bukkit.command.CommandException: Unhandled exception executing command 'wtp' in plugin Needs v1.0
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:42)
at org.bukkit.command.SimpleCommandMap.dispatch(SimpleCommandMap.java:166)
at org.bukkit.craftbukkit.CraftServer.dispatchCommand(CraftServer.java:461)
at net.minecraft.server.NetServerHandler.handleCommand(NetServerHandler.java:818)
at net.minecraft.server.NetServerHandler.chat(NetServerHandler.java:778)
at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:761)
at net.minecraft.server.Packet3Chat.handle(Packet3Chat.java:33)
at net.minecraft.server.NetworkManager.b(NetworkManager.java:229)
at net.minecraft.server.NetServerHandler.a(NetServerHandler.java:112)
at net.minecraft.server.NetworkListenThread.a(NetworkListenThread.java:78)
at net.minecraft.server.MinecraftServer.w(MinecraftServer.java:554)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:452)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:490)
    Caused by: java.lang.NullPointerException
at com.devoverflow.improved.needs.commands.CommandWorldtp.savePlayerInv(CommandWorldtp.java:130)
at com.devoverflow.improved.needs.commands.CommandWorldtp.onCommand(CommandWorldtp.java:60)
at org.bukkit.command.PluginCommand.execute(PluginCommand.java:40)
... 12 more