I has an error if players use ShopGUI+ Plugin to buy or sell, it will error in consoler!  Plugin ShopGUI+:   Error console:  Please help me. Thanks!   class: package com.andrewyunt.townygui.listeners;    import com.andrewyunt.townygui.Menu;  import com.andrewyunt.townygui.TownyGUI;  import com.andrewyunt.townygui.utilities.CommandBuilder;  import com.gmail.filoghost.hiddenstring.HiddenStringUtils;  import org.bukkit.command.CommandSender;  import org.bukkit.entity.Player;  import org.bukkit.event.EventHandler;  import org.bukkit.event.Listener;  import org.bukkit.event.inventory.InventoryClickEvent;  import org.bukkit.inventory.ItemStack;  import org.bukkit.inventory.meta.ItemMeta;    import java.util.List;  import java.util.Set;     public class InventoryListener implements Listener {  @EventHandler public void onInventoryClick(InventoryClickEvent sukien) {      ItemStack item = sukien.getCurrentItem();      Player player = (Player) sukien.getWhoClicked();      if(!(sukien.getInventory().getHolder() == null))         return;      if(item == null || !item.hasItemMeta())         return;      ItemMeta meta = item.getItemMeta();     List&lt;String&gt; lore = meta.getLore();      if(!HiddenStringUtils.hasHiddenString(lore.get(0)))         return;      String action = HiddenStringUtils.extractHiddenString(lore.get(0));      boolean command;      command = action.startsWith("/");      if(!command)         new Menu(player, action);     else {         player.closeInventory();          Set&lt;String&gt; arguments;         try {             arguments = TownyGUI.plugin.commandConfig.getConfig().getConfigurationSection("commands."+ action + ".arguments").getKeys(false);         } catch(NullPointerException e) {             action = action.replace("/", "");             TownyGUI.plugin.server.dispatchCommand(player, action);             sukien.setCancelled(true);             return;         }          new CommandBuilder(arguments, action).beginConversation((CommandSender) player);     }      sukien.setCancelled(true); } }
What are Null Pointer Exceptions (java.lang.NullPointerException) and what causes them?  What methods/tools can be used to determine the cause so that you stop the exception from causing the program to terminate prematurely?
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