catch `nil` strings in text proxies

I ran across a small bug this morning introduced by #1548.

To reproduce:

  1. Search for something with children (an artist from iTunes in my case).
  2. Right-arrow to show the children, then select one *without typing any letters*.
  3. Go to the second pane and call up “Assign Abbreviation…”.

Since there is no search text, there's an exception and the interface sort of loses its mind.