在Angular 8中，HttpTestingController是用于测试HTTP请求的一个辅助类。当在测试中使用HttpTestingController时，有时会出现“无法刷新已取消的请求。”的错误。这通常是由于没有正确处理取消的请求导致的。下面是解决该问题的方法，包含代码示例：

1. 在测试中使用HttpTestingController时，确保在每个测试之后调用HttpTestingController.verify()方法。这将验证是否有未处理的请求，并抛出错误，从而避免出现“无法刷新已取消的请求。”的错误。

import { TestBed, inject } from '@angular/core/testing';
import { HttpClientTestingModule, HttpTestingController } from '@angular/common/http/testing';
import { DataService } from './data.service';

describe('DataService', () => {
  let service: DataService;
  let httpMock: HttpTestingController;

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [HttpClientTestingModule],
      providers: [DataService]
    });

    service = TestBed.inject(DataService);
    httpMock = TestBed.inject(HttpTestingController);
  });

  afterEach(() => {
    // 验证是否有未处理的请求
    httpMock.verify();
  });

  it('should get data', () => {
    const testData = { message: 'Hello World' };

    service.getData().subscribe(data => {
      expect(data).toEqual(testData);
    });

    const req = httpMock.expectOne('/api/data');
    expect(req.request.method).toBe('GET');

    req.flush(testData);
  });
});

2. 在每个测试之前、之后和每个it块之间，确保调用HttpTestingController的相应方法。例如，在beforeEach中使用TestBed.get()方法获取HttpTestingController的实例，并在afterEach中调用HttpTestingController的verify()方法。

import { TestBed, inject } from '@angular/core/testing';
import { HttpClientTestingModule, HttpTestingController } from '@angular/common/http/testing';
import { DataService } from './data.service';

describe('DataService', () => {
  let service: DataService;
  let httpMock: HttpTestingController;

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [HttpClientTestingModule],
      providers: [DataService]
    });

    service = TestBed.get(DataService);
    httpMock = TestBed.get(HttpTestingController);
  });

  afterEach(() => {
    httpMock.verify();
  });

  it('should get data', () => {
    const testData = { message: 'Hello World' };

    service.getData().subscribe(data => {
      expect(data).toEqual(testData);
    });

    const req = httpMock.expectOne('/api/data');
    expect(req.request.method).toBe('GET');

    req.flush(testData);
  });
});

通过以上方法，您应该能够解决“Angular 8 HttpTestingController错误 - '无法刷新已取消的请求。'”的问题，并正确处理取消的请求。