Angular 服务器端渲染的一个错误消息 - No provider for InjectionToken REQUEST

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 Application 里：

import { Request } from 'express';
import { REQUEST } from '@nguniversal/express-engine/tokens';
@Injectable()
export class RequestService {