I have a record choice set and it is used as a choice in radio button in a screen component. The other choice is None.
On the basis of number of records available in record choice set, if only one record is available then I want to default that as a selected choice. If there are more than one records available then I want to default None choice and let user select the required choice.
 
Tried approach - created a formula variable with if logic. In that checked if record choice set contains semi-colon means more than one records are available(CONTAINS({!tField},";")). If true then None choice, if false record choice set as a choice.
 
This formula is not working any other approaches is appreciated. Thanks

I got stucked in PD2  Advanced apex specialist challenge2 - 
Update the codebase to use best practices.

I am getting below error while clcking check challenge. I have checked validation rules in my org but still same error. Thanks in advance.