how to remove "null" from a repeating subform value?

I have a repeating subform (Post) that has a text field (txtNotes) and I am trying to remove the 'null' value, how do I achieve that?

nTotal = 0;

for (var i = 0; i < _Post.count; i++)
{
var oItem = Page1.resolveNode("Post[" + i + "]");

nTotal += oItem.Staff.txtNotes.rawValue + "\n";
}