Fix NPE in Nukkit disconnect handling (#2269)

nukkit/src/main/java/me/lucko/luckperms/nukkit/listeners/NukkitConnectionListener.java

@@ -188,6 +188,12 @@ public class NukkitConnectionListener extends AbstractConnectionListener impleme
     @EventHandler(priority = EventPriority.MONITOR)
     public void onPlayerQuit(PlayerQuitEvent e) {
         final Player player = e.getPlayer();
+
+        // https://github.com/lucko/LuckPerms/issues/2269
+        if (player.getUniqueId() == null) {
+            return;
+        }
+
         handleDisconnect(player.getUniqueId());
 
         // perform unhooking from nukkit objects 1 tick later.