I'm new to VisualForce, so likely missing something simple. I have a simple flow that looks up a contact, and if it finds a match writes the matching contact.id to a text variable. I know this works because in a later step I refer to the variable to update the existing record. But I want this workflow to go to the lookup record when it's done. It sounds like the best way to do that is setting the finishlocation to a page reference that pulls the variable from the workflow. But that page reference keeps coming up null--so the variable value isn't getting pulled into the controller.

Here's my page. "HES_signup_test" is the name of the otherwise working workflow:
<apex:page controller="HESFlowToSchedule">
 <flow:interview name="HES_signup_test" interview="{!myflow}" finishlocation="{!ContactPage}">
  </flow:interview>
</apex:page>

Here's my controller, copied from an example at https://developer.salesforce.com/forums/?id=906F0000000QtIxIAK. "PassToSO" is the flow variable name.
public class HESFlowToSchedule {
    public Flow.Interview.HES_signup_test myflow {get; set;}
    public PageReference ContactPage {
        get {
            PageReference pageRef = new PageReference('/' + flowContactId );
            pageRef.setRedirect(true);
            return pageRef;
        }
        set { ContactPage = value; }
    }
    public String flowContactId {
        get {
            String strTemp = '';
            if(myflow != null) {
                strTemp = myflow.PassToSO; //also tried string.valueOf(myflow.getVariableValue('PassToSO'));
            }
            return strTemp;
        }
        set { flowContactId = value; }
    }
}
In this example, logs show StrTemp ends up null, even though PassToSO is successfully used in the workflow. PassToSO is set for Input and Output use.

I also tried this second construction, from https://developer.salesforce.com/docs/atlas.en-us.208.0.pages.meta/pages/pages_flows_advanced.htm:
public class HESFlowToSchedule {
    public Flow.Interview.HES_signup_test myflow {get; set;}
    public PageReference getcontactPage() {
        String apexPassToSO;
        if (myFlow==null) { return new PageReference('/0030q000001xgYH');}
        else {
            apexPassToSO = (String) myFlow.getVariableValue('PassToSO');
            if (apexPassToSO==null) {return new PageReference('/0033700000SEGA8');}
           else {
                return new PageReference('/' + apexPassToSO);
            }
        }
    }
and it pulled a null value.

Then I tried this, from https://help.salesforce.com/articleView?id=pages_flows_getting_values.htm&language=en_US&type=0, with same failure.
public class GetPassToSO {
    public Flow.Interview.HES_signup_test myflow { get; set; }
    public String apexPassToSO;
    public String getApexPassToSO() {
        return myflow.PassToSO;
    }
}
What am I missing?  Thanks for any clues!