Fixed NPE when clicking non gui items

core/src/main/java/dev/triumphteam/gui/guis/GuiListener.java

@@ -1,18 +1,18 @@
 /**
  * MIT License
- *
+ * <p>
  * Copyright (c) 2021 TriumphTeam
- *
+ * <p>
  * Permission is hereby granted, free of charge, to any person obtaining a copy
  * of this software and associated documentation files (the "Software"), to deal
  * in the Software without restriction, including without limitation the rights
  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  * copies of the Software, and to permit persons to whom the Software is
  * furnished to do so, subject to the following conditions:
- *
+ * <p>
  * The above copyright notice and this permission notice shall be included in all
  * copies or substantial portions of the Software.
- *
+ * <p>
  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
@@ -173,7 +173,9 @@ public void onGuiOpen(final InventoryOpenEvent event) {
     private boolean isGuiItem(@Nullable final ItemStack currentItem, @Nullable final GuiItem guiItem) {
         if (currentItem == null || guiItem == null) return false;
         // Checks whether or not the Item is truly a GUI Item
-        return ItemNbt.getString(currentItem, "mf-gui").equals(guiItem.getUuid().toString());
+        final String nbt = ItemNbt.getString(currentItem, "mf-gui");
+        if (nbt == null) return false;
+        return nbt.equals(guiItem.getUuid().toString());
     }
 
 }