Why are you using a HTML widget and putting another iframe in it?
A HTML widget is a speciel element that already has an iframe in it.
Using the link that Hans has supplied above (again here if you don't want to scroll up)
You will see how you can drill down into a HTML widget to get to the inner elements.
But, really I don't know why you need to have an iframe in an iframe. Gonna make it so laborious drilling down to get to it. Why not use another element? Or just use the HTML widget for what you want.