#vulnerable code 
                                private void register(List<URL> registryUrls, URL serviceUrl) {
        for (URL url : registryUrls) {
            // 根据check参数的设置，register失败可能会抛异常，上层应该知晓
            RegistryFactory registryFactory = ExtensionLoader.getExtensionLoader(RegistryFactory.class).getExtension(url.getProtocol());
            Registry registry = registryFactory.getRegistry(url);
            registry.register(serviceUrl);
        }
    } 
                            #location 5 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                private void register(List<URL> registryUrls, URL serviceUrl) {

        for (URL url : registryUrls) {
            // 根据check参数的设置，register失败可能会抛异常，上层应该知晓
            RegistryFactory registryFactory = ExtensionLoader.getExtensionLoader(RegistryFactory.class).getExtension(url.getProtocol());
            if (registryFactory == null) {
                throw new MotanFrameworkException(new MotanErrorMsg(500, MotanErrorMsgConstant.FRAMEWORK_REGISTER_ERROR_CODE,
                        "register error! Could not find extension for registry protocol:" + url.getProtocol()
                                + ", make sure registry module for " + url.getProtocol() + " is in classpath!"));
            }
            Registry registry = registryFactory.getRegistry(url);
            registry.register(serviceUrl);
        }
    }