Bukkit

Description:

Using Persistent-Nicknames causes some other plugins to crash or produce errors for players who do not have a nickname set by Persistent-Nicknames.

Reproduction:

1. Use Persistent-Nicknames with a plugin that uses display names (I use CommandBook).
2. Log in with a user that does not have a nickname set.
3. Observe other plugins producing errors.

Details:

I reproduce here the code from your player login handler:

  @EventHandler(priority=EventPriority.NORMAL)
  public void renameOnLogin(PlayerLoginEvent event)
  {
    Player player = event.getPlayer();
    player.setDisplayName(getConfig().getString("nicknames." + player.getName()));
  }

If a player does not have a nickname defined in the config file, then getConfig().getString() returns null. The null value is stored by CraftBukkit and returned by getDisplayName(). Other plugins are not expecting a null value here, and this results in undefined behaviour.

A display name should NEVER be set to null. I consider it a Bukkit bug that setDisplayName() allows you to do this.

Update: Bukkit have fixed this on their end.

https://bukkit.atlassian.net/browse/BUKKIT-5254