无法读取在Angular和Firebase Auth中未定义的属性'user‘

import { Component } from '@angular/core';
import { AngularFireAuth } from '@angular/fire/auth';

@Component({
  selector: 'app-example',
  template: `
    <div *ngIf="user">
      Welcome, {{ user.displayName }}!
    </div>
  `,
})
export class ExampleComponent {
  user: firebase.User | null;

  constructor(private afAuth: AngularFireAuth) {
    this.afAuth.authState.subscribe((user) => {
      this.user = user;
    });
  }
}