destroy () {
    document.removeEventListener('click', this._onClick)
    document.removeEventListener('keyup', this._onKeyUp)
    document.removeEventListener('blur', this._onBlur)
    this._onEvent({ target: document.body }) // calling _onEvent with body element in the hope that the FocusTracker is added to an element inside the body tag
  }
Removes the event listeners on the 'document' object
that were added to track the focus of the given 'target'