Fetching data in stateless form from other form

Fetching data in stateless form from other form

I have generated customer form and stateless customer form. I have generated one field in stateless form which has a condition.

Suppose anybody enters sMobile NO in phone field of stateless customer form it shows name in sName field of stateless customer form provided that phone number is already entered in Mobile No field of customer form and name also entered against that Mobile No field in name field of customer form. I have generated following script and put it in on user input of sMobile NO of stateless customer form. It works very fine if data is already available in customer form. But shows error if data is not available.

Contactno  =  Customer  [Mobile_No == input.sMobile_NO];
input.sName = Contactno.Name;

I want to use condition  as follow:
On entering number in sMobile NO field, script on user input should be such that
If data is available in customer form it should show name in sName field and show other fields
Else it should direct to customer form to fill data.

How to do?