title: is it necessary to set Fragment interface listeners to null on detach?
body: <p>In Fragment examples involving callbacks, usually they assign the listener in the onAttach method and then set the listener to null in the onDetach method.</p>
  
  <p>Is this last part necessary? Doesn't the listener automatically get set to null when the fragment is detached / destroyed? Or are there situations where you might detach the fragment and re-attach it somewhere else, and you wouldn't want the callback pointing to anything in particular until it gets reattached to something?</p>