the selection returns empty

[Roger Eller]

I am probably missing something simple, but if I select a line in a list
field, then click a button that puts the selection into a variable, it
works the first time.  However, if focus changes to another object, but the
selection is still visibly shown in the list field, the button now returns
empty.

How can I get my button to always return the visibly selected content of
this specific list field?

~Roger