TypeError: Невозможно прочитать свойство 'uid' из null в SwitchMapSubscriber.project

Question

TypeError: Cannot read property 'uid' of null
     at SwitchMapSubscriber.project (auth.service.ts:37)
     at SwitchMapSubscriber.push../node_modules/rxjs/_esm5/internal/operators/switchMap.js.SwitchMapSubscriber._next
 (switchMap.js:34)
     at SwitchMapSubscriber.push../node_modules/rxjs/_esm5/internal/Subscriber.js.Subscriber.next
 (Subscriber.js:54)

Вот код

import { Injectable } from '@angular/core';
import { AngularFireAuth } from 'angularfire2/auth';
import * as firebase from 'firebase';
import { Observable } from 'rxjs/Observable';
import { ActivatedRoute } from '@angular/router';
import { UserService } from './user.service';
import { AppUser } from './models/app-user';
import 'rxjs/add/operator/switchMap';

@Injectable()
export class AuthService {

  user$: Observable<firebase.User>;

  constructor(
    private userService:UserService,
    private afAuth: AngularFireAuth,
    private route:ActivatedRoute
  ) { 
    this.user$ = afAuth.authState;
  }

  login(){
    let returnUrl = this.route.snapshot.queryParamMap.get('returnUrl') || '/';
    localStorage.setItem('returnUrl',returnUrl);
    this.afAuth.auth.signInWithRedirect(new firebase.auth.GoogleAuthProvider());
  }

  logout(){
    this.afAuth.auth.signOut();
  }

  get appUser$() : Observable< AppUser >{
    return this.user$
      .switchMap(user => this.userService.get(user.uid).valueChanges());
  }

}

UserService

import { Injectable } from '@angular/core';
import { AngularFireDatabase, AngularFireObject } from 'angularfire2/database';
import * as firebase from 'firebase';
import { AppUser} from './models/app-user';

@Injectable()
export class UserService {

  constructor(private db:AngularFireDatabase) { }

  save(user: firebase.User){
    this.db.object('/users/' +user.uid).update({
      name:user.displayName,
      email:user.email
    });
  }

  get(uid: string): AngularFireObject<AppUser>{
     return this.db.object('/users/'+ uid);
  }

}

Answer 1

get appUser$(): Observable<AppUser>{
      return this.user$
      .switchMap(user => {
        if(user) return this.userService.get(user.uid);
          return Observable.of(null);
      })
    }

Answer 2

Просто добавьте, если проверка, если пользователь доступен, то вы запрашиваете, используя идентификатор,

get appUser$(): Observable<AppUser>{
      return this.user$
      .switchMap(user => {
        if(user) return this.UserService.get(user.uid)
          return Observable.of(null);
      })
    }