Validate properly the 'quantity' widget on POS

When clear the 'quantity' widget on POS and immediatelly activate it,
an error was happening.
- Before activate this widget, check if is valid value.

Change-Id: I4fe947afa3fb735633f3f5749f3dec31083fc264
(cherry picked from commit 8e6247316d6326545d01ec1a9b1e8e4193d88304)