Problem with getValue() in Numberfield

Hi,

I have a set of Numberfields in my app and I want to show an alert for the user when he tapes a wrong value, for example: '123((7' . In that case, getValue() results in 'null' but when the Numberfield is empty getValue() results in a 'null' also, nevertheless 'empty value' must be allowed in my app.

How can I differentiate between 'empty' an 'wrong' value?

Thanks in advance.