i have this same error since i updated to the last version of woocommerce,this error always show when i add a new order, i’ve added the follow code on the woocommerce-product-stock-alert/classes/class-woo-product-stock-alert-admin.php on line 231 of wc_get_product to $product_obj = new WC_Product( $post_id ); this works for me
