The ListView
recycles the Views
that are contained in the list. The Views themselves are merely visual representations of the data that the provided Adapter gives it. The ListView itself does absolutely nothing to the actual data that it is showing. You don't have to nullify anything.
The reason this happens is in ListViews, the list items are almost all exactly the same (usually). So when a list item goes out of sight, the ListView will take the View and give it to the adapter. The adapter will then fill the View with the appropriate data and give it back to the ListView. The ListView will then stick it to the bottom (or top) of the visible part of the list. This cuts down on overhead of inflating new Views. It limits memory usage of keeping View objects in memory. Makes an overall smoother scroll.