Fix LT-14050: Instant crash with Assign Features to Phonemes

Also added code to
1. change the "Target Field" label to "Target Feature";
2. change the standard BE label to be "Either select the target feature
   and a value to change it to or choose phonological features";
3. disable the target combo whenever the user uses the phonological
   feature chooser;
4. fix crash if user did not choose anything in chooser.

Change-Id: Idedddb28e36a33cc4aded2481e87e46293dfdfa6