未处理的承诺拒绝：无法在“IDBDatabase”angularfire 上执行“交易”

Question

我有角度，angularfire2 我有错误

未处理的承诺拒绝：无法在“IDBDatabase”上执行“交易”：数据库连接正在关闭。; 区域： ; 任务： Promise.then ；值：DOMException：无法在“IDBDatabase”上执行“事务”：数据库连接正在关闭。

我的简单代码是

import { Component } from '@angular/core';
import { AngularFirestore } from '@angular/fire/firestore';
import { Observable, from } from 'rxjs';
import { map } from 'rxjs/operators';
import { Trip } from '../models/Trip';

@Component({
  selector: 'app-root',
  template: `
  <ul>
    <li *ngFor="let item of items | async">
      {{ item.tripname }}
    </li>
  </ul>
  `
})
export class TestpageComponent {

  items: Observable<any[]>;
  constructor(db: AngularFirestore) {
    this.items = db.collection('trips').snapshotChanges().pipe(
      map(actions => actions.map(a => {
        const data = a.payload.doc.data() as Trip;
        const id = a.payload.doc.id;
        return { id, ...data };
      }))
    );
  }

  ngOnInit() {
  }

}

请帮忙