unapply (handler) {
    super.unapply(handler, () => {
      [...this.stack].reverse().forEach((stackItem) => {
        console.assert(stackItem, 'stack item should not be null')
        stackItem && stackItem.unapply(handler)
      })
    })
  }
Runs "unapply" on all subcommands.
@param {module:history.HistoryEventHandler} handler
@fires module:history~Command#event:history
@returns {void}