Failed to execute 'open' on 'XMLHttpRequest': Invalid URL

我遇到了运行时错误：

执行 'open' on 'XMLHttpRequest' 失败：无效的 URL

错误信息：执行 'open' on 'XMLHttpRequest' 失败：无效的 URL

位于 http://localhost:8100/build/polyfills.js:3:2793

在 XMLHttpRequest.open (eval at s (http://localhost:8100/build/polyfills.js:2:29793), :3:31)

import { Injectable } from '@angular/core';
import { Http, Headers } from '@angular/http';
import 'rxjs/add/operator/map';
@Injectable()
export class Reviews {
  data: any;
  constructor(public http: Http) {
    this.data = null;
  }
  getReviews(){
    if (this.data) {
      return Promise.resolve(this.data);
    }
    return new Promise(resolve => {
      this.http.get('http://localhost:app/api/reviews')
        .map(res => res.json())
        .subscribe(data => {
          this.data = data;
          resolve(this.data);
        });
    });
  }
  createReview(review){
    let headers = new Headers();
    headers.append('Content-Type', 'application/json');
    this.http.post('http://localhost:app/api/reviews', JSON.stringify(review), {headers: headers})
      .subscribe(res => {
        console.log(res.json());
      });
  }
  deleteReview(id){
    this.http.delete('http://localhost:app/api/reviews/' + id).subscribe((res) => {
      console.log(res.json());
    });    
  }
}