SPIGOT-6666: NPE when deserializing bundle

By: md_5 <git@md-5.net>

paper-server/src/main/java/org/bukkit/craftbukkit/inventory/CraftMetaBundle.java

@@ -62,7 +62,7 @@ public class CraftMetaBundle extends CraftMetaItem implements BundleMeta {
         if (items != null) {
             for (Object stack : items) {
                 if (stack instanceof ItemStack) {
-                    this.items.add((ItemStack) stack);
+                    addItem((ItemStack) stack);
                 }
             }
         }