Update date field from another model

Questions
,
3

Yes, the Timeline model is related to Program model with Program__c, and both models have the Mail_Date__c fields

Tried programModelRow.Mail_Date__c in place of Program__r.Mail_Date__c and still not working.

Running a test, I am able to update the Timeline field to ‘null’ without problem (below)
var params = arguments[0], $ = skuid.$;
var model = skuid.$M(‘Timeline’);
var row = model.getFirstRow();
model.updateRow(row,{


Mail_Date__c: null 
});