Sometimes a custom dialog launched from script will return an empty string ("") in its .Response property if the user clicks away from the Sansar client and then returns a few seconds later to click one of the two buttons.
This is fairly repeatable, though the exact recipe is not exactly certain to me. I find I wait at least 3 seconds before clicking on the client again. And I click somewhere in the viewer that is not part of the dialog box. I don't know if these are causally related or just coincidences.