when variant is new, set the product on it (that'll set ownership)

src/elements/CommerceProduct.php

@@ -448,6 +448,7 @@ private function _parseVariants($event): void
             // Create a new variant, or find an existing one to edit
             if (!isset($variants[$sku])) {
                 $variants[$sku] = new VariantElement();
+                $variants[$sku]->product = $element;
             }
 
             // We are going to handle stock after the product and variants save