Rxjs异步拦截器-不断获取错误

我需要在我的拦截器中为请求添加一个令牌。我一直在犯错误

core.js:1673 ERROR TypeError: You provided 'undefined' where a stream was expected. You can provide an Observable, Promise, Array, or Iterable.
    at subscribeTo (subscribeTo.js:41)
    at subscribeToResult (subscribeToResult.js:11)
    at MergeMapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/mergeMap.js.MergeMapSubscriber._innerSub (mergeMap.js:73)
    at MergeMapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/mergeMap.js.MergeMapSubscriber._tryNext (mergeMap.js:68)
    at 

在AuthService.ts中,我有

  getToken2() {
    if (firebase.auth().currentUser) {
      return from(firebase.auth().currentUser.getIdToken());
    } else {
      return null;
    }
  }

import {
  HttpInterceptor,HttpRequest,HttpHandler,HttpEvent} from "@angular/common/http";
import { Observable } from "rxjs";
import { mergeMap, switchMap, flatMap } from "rxjs/operators";
import { Injectable } from "@angular/core";
import { AuthService } from "../auth/auth.service";

@Injectable()
export class AuthInterceptor implements HttpInterceptor {
  constructor(private authService: AuthService) {}

  intercept(
    req: HttpRequest<any>,
    next: HttpHandler
  ): Observable<HttpEvent<any>> {
    const tokenObservable = this.authService.getToken2(); // if there is a user logged in
    if (tokenObservable) {
      tokenObservable.pipe(
        switchMap(token => {
          const copiedReq = req.clone({
            params: req.params.set("auth", token)
          });
          return next.handle(copiedReq);
        })
      );
    } else {
      return next.handle(req);
    }
  }
}