convertView parameter may be a
recycled view (for example, after scrolling down, the top rows become
invisible, so their
View objects are not destroyed, but
recycled and passed as parameters for reuse).
However, the very first time a draw request comes, there is no view
(e.g. the first time the screen with the list is loaded). Hence, in this
convertView has no value because nothing has been
recycled (it is
null), in which case you must create the rows
using the inflater.