The problem in this bug report is already analyzed: There is a general dispatcher for the requested tasks of ifconfig. After fulfilling these tasks, ifconfig goes on to show information, but then, the interface doesn't exist anymore.

You have to think of a clean way how to circumvent this, i.e. either catch non-existing devices before the task of ifconfig is done, or do a differentiation afterwards such that the error is not printed anymore.
ifconfig is a very complex tool, so expect a high difficulty from this task, not in the coding part, but in the analyzation part!

http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=46579