dropdown.dispose() does not remove event listeners

Created by: jirikanda

Dropdown has a dispose() function which contains:

super.dispose()
EventHandler.off(this._element, EVENT_KEY)
...

Unfortunatelly, in the base class BaseComponent there is the dispose() function which contains

Data.removeData(this._element, this.constructor.DATA_KEY)
this._element = null

When dropdown dispose() function is called

1. this._element is set to null in the super.dispose()
2. EventHandler.off function(...) cannot remove event listener because the this._element is null.