As the error states you are passing string while the function expects an object
PHP Code:
$preorder_details = \XLite\Core\Database::getRepo('XLite\Module\Webkulsoftware\PreorderSales\Model\Preorders')->getPreOrder('',$product_id);
This should works
PHP Code:
$cnd = new \XLite\Core\CommonCell;
$preorder_details = \XLite\Core\Database::getRepo('XLite\Module\Webkulsoftware\PreorderSales\Model\Preorders')->getPreOrder($cnd,$product_id);