无法清除ngOnDestroy（）中的Interval

Question

我正在尝试使用setInterval（）监视window.open（）打开的选项卡的关闭事件。当我尝试在ngOnDestroy（）中清除clearInterval时，它总是告诉我微调是未定义的。有建议吗？

timmer: any;
newWindow: any;

ngOnDestroy() {
    this.newWindow.close(); // newWindow is always undefined...
    clearInterval(this.timmer) // timmer is always undefined...
  }

// this I am calling an api to get the new window url and use window.open to open it...

public openWindow() {
    this.claimsService.getUrl(this.id).subscribe(
      response => {
        this.newWindow = window.open(response.url);
        this.timmer = setInterval(() => {
          if (newWindow.closed) {
            clearInterval(loop);
          }
      },
      error => {
        //...error handler...
      }
    );
  }