Application is crashing when manually changing caret position

Steps to reproduce:
- set US phone format as pattern: (###) ###-####
- apply behavior on textbox and run application.
- type in 5555
- behavior will change text to (555) 5
- now press Home button (or change caret position with mouse click) 
- press any key (5 for example)
- application will crash because behavior is trying to set negative number to SelectionStart property.
