startPollingTabs() {
    heartBeat = window.setInterval(() => this.addInterval(), this.heartBeatInterval);
  }
Poll all tabs for their status - OPENED / CLOSED
An interval is created which checks for last and current status.
There's a property on window i.e. `closed` which returns true for the closed window.
And one can see `true` only in another tab when the tab was opened by the same `another` tab.