注销时AngularFire2取消订阅，未关闭订阅时出现数据库权限错误

import { Component, OnDestroy } from '@angular/core';
import { AngularFireDatabase, AngularFireObject } from '@angular/fire/database';
import { Subscription } from 'rxjs';

@Component({
  selector: 'app-example',
  template: `
    <div *ngIf="data$ | async as data">
      {{ data | json }}
    </div>
  `,
})
export class ExampleComponent implements OnDestroy {
  data$: AngularFireObject<any>;
  subscription: Subscription;

  constructor(private db: AngularFireDatabase) {
    this.data$ = db.object('path/to/data');
    this.subscription = this.data$.valueChanges().subscribe();
  }

  ngOnDestroy() {
    this.subscription.unsubscribe();
  }
}