要解决“NullInjectorError: No provider for HttpClient!”错误，您可以按照以下步骤进行操作：

1. 确保您在使用HttpClient之前已经导入了HttpClientModule：

import { HttpClientModule } from '@angular/common/http';

@NgModule({
  imports: [HttpClientModule],
  ...
})
export class AppModule { }

2. 在您的服务（或组件）的测试文件中，添加HttpClientTestingModule：

import { HttpClientTestingModule } from '@angular/common/http/testing';

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

3. 如果您在测试前需要使用HTTP请求进行设置，请使用HttpTestingController来拦截请求并提供响应。例如：

import { HttpClientTestingModule, HttpTestingController } from '@angular/common/http/testing';

describe('YourService', () => {
  let service: YourService;
  let httpTestingController: HttpTestingController;

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

    service = TestBed.get(YourService);
    httpTestingController = TestBed.get(HttpTestingController);
  });

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

  it('should make a GET request', () => {
    const testData = { message: 'test' };

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

    const req = httpTestingController.expectOne('your-url');
    expect(req.request.method).toEqual('GET');

    req.flush(testData);
  });

});

以上是解决“NullInjectorError: No provider for HttpClient!”错误的一般步骤。根据您的具体代码和测试设置，您可能需要进行一些适应性调整。