Fixed #16659

CHANGELOG.md

@@ -11,6 +11,7 @@
 - Fixed a bug where user addresses weren’t getting restored when soft-deleted users were restored. ([#16636](https://github.com/craftcms/cms/pull/16636))
 - Fixed a bug where pressing the “New entry” button multiple times quickly would create multiple nested entries, circumventing the “Max Entries” settings. ([#16642](https://github.com/craftcms/cms/issues/16642))
 - Fixed a bug where Link fields without values were always getting marked as dirty when making another change to the element. ([#16649](https://github.com/craftcms/cms/pull/16649))
+- Fixed an error that could occur when programmatically duplicating a nested element for a new site. ([#16659](https://github.com/craftcms/cms/issues/16659))
 - Fixed a potential phishing attack vector.
 - Fixed a styling issue.
 

src/base/NestedElementTrait.php

@@ -105,18 +105,26 @@ public static function eagerLoadingMap(array $sourceElements, string $handle): a
     public bool $updateSearchIndexForOwner = false;
 
     /**
-     * @var ElementInterface|false The primary owner element, or false if [[primaryOwnerId]] is invalid
+     * @var ElementInterface|false|null The primary owner element, or false if [[primaryOwnerId]] is invalid
      * @see getPrimaryOwner()
      * @see setPrimaryOwner()
      */
-    private ElementInterface|false $_primaryOwner;
+    private ElementInterface|false|null $_primaryOwner = null;
 
     /**
-     * @var ElementInterface|false The owner element, or false if [[ownerId]] is invalid
+     * @var ElementInterface|false|null The owner element, or false if [[ownerId]] is invalid
      * @see getOwner()
      * @see setOwner()
      */
-    private ElementInterface|false $_owner;
+    private ElementInterface|false|null $_owner = null;
+
+    public function __clone(): void
+    {
+        parent::__clone();
+
+        $this->_primaryOwner = null;
+        $this->_owner = null;
+    }
 
     /**
      * @inheritdoc