Something went wrong while saving this configuration

When i try to save theme configuration of the theme, appears the error:
“Notice: Trying to get property ‘attributes’ of non-object in /bitnami/magento/vendor/magento/module-webapi/Model/Config/Converter.php on line 51”

            /** @var DOMElement $service */
        $service = $route->getElementsByTagName('service')->item(0);
       line 51-> $serviceClass = $service->attributes->getNamedItem('class')->nodeValue;
        $serviceMethod = $service->attributes->getNamedItem('method')->nodeValue;
        $soapMethod = $serviceMethod;
        if ($soapOperationNode = $route->attributes->getNamedItem('soapOperation')) {
            $soapMethod = trim($soapOperationNode->nodeValue);
        }
        $url = trim($route->attributes->getNamedItem('url')->nodeValue);
        $version = $this->convertVersion($url);