我正在嘗試使用 Ionic 和 AngularFire 從 Firebase Firestore 數據庫中獲取我用戶的用戶名。我正在使用該valueChanges()方法來獲取 observable，然后嘗試使用異步管道解釋 observable。但是，當我運行代碼時，出現以下錯誤：錯誤但是，當我記錄 observable 時，它顯示如下：可觀察的profile.page.ts：import { Component, OnInit } from '@angular/core';import { AngularFireAuth } from '@angular/fire/auth';import { Router } from '@angular/router';import { UserService } from '../user.service'import { AngularFirestore, AngularFirestoreCollection } from '@angular/fire/firestore'import { Observable } from 'rxjs';@Component({  selector: 'app-profile',  templateUrl: './profile.page.html',  styleUrls: ['./profile.page.scss'],})export class ProfilePage implements OnInit {  constructor(public afAuth: AngularFireAuth, public router: Router, public user: UserService, public db: AngularFirestore) {   }  ngOnInit() {  }  logout() { this.afAuth.auth.signOut(); this.router.navigate(['/login']);}}profile.page.html： <ion-content>  <ion-grid>      <ion-row justify-content-center align-items-center style="height: 50%">    <ion-button color="danger" size="small" shape="round" (click)="logout()">Logout</ion-button>    <p *ngFor="let users of (username | async)">{{ users.username }}</p>    </ion-row>    </ion-grid></ion-content>  在此先感謝您的幫助。