无法读取null的属性“subscribe”

  getDefaultNavbarStyle(templateId): any {
    let template = null;
    switch (templateId) {
      case ActiveTemplates.Default:
        this.httpClient.get(this.DEFAULT_TEMPLATE_PATH).subscribe(response => {
          template = response;
        });
        break;
      case ActiveTemplates.Quick:
        this.httpClient.get(this.QUICK_TEMPLATE_PATH).subscribe(response => {
          template = response;
        });
        break;
      case ActiveTemplates.Front:
        this.httpClient.get(this.FRONT_TEMPLATE_PATH).subscribe(response => {
          template = response;
        });
        break;
      default:
        break;
    }
    return template;
  }

this.defaultNavbarStyleSubscription = this.builderNavbarService.getDefaultNavbarStyle(this.navbarTemplate).subscribe(response => {
  if (response) {
    this.defaultNavbarStyle = response;
  }
});

但是,我不断得到以下错误:

ERROR TypeError: Cannot read property 'subscribe' of null

我能做些什么来修复它?我已将模板初始化为null,但在switch语句中设置了它的值。