错误消息：

NullInjectorError: R3InjectorError(AppServerModule)

WindowRef -> InjectionToken SERVER_REQUEST_URL -> InjectionToken SERVER_REQUEST_URL -> InjectionToken SERVER_REQUEST_URL -> InjectionToken REQUEST]:

NullInjectorError: No provider for InjectionToken REQUEST!

Angular universal engine 的文档：

https://github.com/angular/universal/tree/master/modules/express-engine

服务器端的 HTTP request 调用，通过 Injection token 注入到 SSR Application 里：

import { Request } from 'express';
import { REQUEST } from '@nguniversal/express-engine/tokens';
@Injectable()
export class RequestService {
  constructor(@Inject(REQUEST) private request: Request) {}
这个 token 在客户端环境下不可用。
相关错误：https://github.com/angular/universal-starter/issues/423
https://gitter.im/angular/universal/archives/2018/06/17
REQUEST and RESPONSE won't exist when you run in the browser context, even when you are running the SSR server

Use @Optional and take care to check whether you've got it, and consider using isServerPlatform to detect when you are running on the server side
在浏览器上下文环境运行时，REQUEST 和 RESPONSE 不可用。使用 @Optional 来避免 NullInjectorError 错误，以及使用 isServerPlatform 检测当前的运行环境。
相关 issue：https://github.com/SAP/spartacus/issues/11016
The SERVER_REQUEST_ORIGIN and SERVER_REQUEST_URL should not be provided in CSR by design.
SERVER_REQUEST_ORIGIN 在文件 ssr-providers.ts 里提供实现：