SPIGOT-6529: Fix BundleMeta#setItems

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

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

@@ -116,7 +116,7 @@ public class CraftMetaBundle extends CraftMetaItem implements BundleMeta {
 
     @Override
     public void setItems(List<ItemStack> items) {
-        items = null;
+        this.items = null;
 
         if (items == null) {
             return;