"Failed to handle packet for" while checking ip on join

Hello! I just ran into a problem that I can solve somehow. It appear when I check the IP when a player join the server. It's in PlayerJoinEvent, here's my code:

Code:java

 
if(Referral.ips.contains(player.getAddress().getHostString()))
{
System.out.println("asd");
return;
}
 
if(!player.hasPlayedBefore())
{
try{
Inventory inv = Bukkit.createInventory(player, 9);
inv.addItem(new ItemStack(Material.DIAMOND, 1));
player.openInventory(inv);
 
Referral.ips.add(player.getAddress().getHostString());
plugin.getConfig().set("IPs", Referral.ips);
plugin.saveConfig();
System.out.println(player.getAddress().getHostString());
}catch(Exception ex)
{
ex.printStackTrace();
}
}else{
System.out.println(player.getAddress().getHostString());
System.out.println("Lel played before");
}
 

It also appears when I try to add the ip to the config stringlist.
Here's the complete error, I tried on not localhost too:

Code:

[10:40:20 WARN]: Failed to handle packet for /127.0.0.1:49768
java.lang.IllegalArgumentException: Listener already listening
        at net.minecraft.server.v1_7_R1.Container.addSlotListener(Container.java
:53) ~[craftbukkit-1.7.2.jar:git-Bukkit-1.6.4-R2.0-39-g68b702f-b2954jnks]
        at net.minecraft.server.v1_7_R1.EntityPlayer.syncInventory(EntityPlayer.
java:152) ~[craftbukkit-1.7.2.jar:git-Bukkit-1.6.4-R2.0-39-g68b702f-b2954jnks]
        at net.minecraft.server.v1_7_R1.PlayerList.a(PlayerList.java:131) ~[craf
tbukkit-1.7.2.jar:git-Bukkit-1.6.4-R2.0-39-g68b702f-b2954jnks]
        at net.minecraft.server.v1_7_R1.LoginListener.c(LoginListener.java:78) ~
[craftbukkit-1.7.2.jar:git-Bukkit-1.6.4-R2.0-39-g68b702f-b2954jnks]
        at net.minecraft.server.v1_7_R1.LoginListener.a(LoginListener.java:42) ~
[craftbukkit-1.7.2.jar:git-Bukkit-1.6.4-R2.0-39-g68b702f-b2954jnks]
        at net.minecraft.server.v1_7_R1.NetworkManager.a(NetworkManager.java:149
) ~[craftbukkit-1.7.2.jar:git-Bukkit-1.6.4-R2.0-39-g68b702f-b2954jnks]
        at net.minecraft.server.v1_7_R1.ServerConnection.c(SourceFile:134) [craf
tbukkit-1.7.2.jar:git-Bukkit-1.6.4-R2.0-39-g68b702f-b2954jnks]
        at net.minecraft.server.v1_7_R1.MinecraftServer.u(MinecraftServer.java:6
51) [craftbukkit-1.7.2.jar:git-Bukkit-1.6.4-R2.0-39-g68b702f-b2954jnks]
        at net.minecraft.server.v1_7_R1.DedicatedServer.u(DedicatedServer.java:2
50) [craftbukkit-1.7.2.jar:git-Bukkit-1.6.4-R2.0-39-g68b702f-b2954jnks]
        at net.minecraft.server.v1_7_R1.MinecraftServer.t(MinecraftServer.java:5
41) [craftbukkit-1.7.2.jar:git-Bukkit-1.6.4-R2.0-39-g68b702f-b2954jnks]
        at net.minecraft.server.v1_7_R1.MinecraftServer.run(MinecraftServer.java
:453) [craftbukkit-1.7.2.jar:git-Bukkit-1.6.4-R2.0-39-g68b702f-b2954jnks]
        at net.minecraft.server.v1_7_R1.ThreadServerApplication.run(SourceFile:6
17) [craftbukkit-1.7.2.jar:git-Bukkit-1.6.4-R2.0-39-g68b702f-b2954jnks]
[10:40:20 INFO]: addemod lost connection: Internal server error
[10:40:20 INFO]: addemod left the game.

 

what about removing lines about inventory edit.

 

Really? That was the error? -_-
I feel stupid now. But I can't see why I am not allowed to do that? It works in other plugins :s

Edit, fixed. Just noticed that I can't do it while the player is connecting. Thanks btw. :3
Used a timer to do it 2 seconds later.