Add to cart is not working for products with selectable features

Issue-1: Steps to regenerate:

1. Visit the https://demo-trunk.ofbiz.apache.org/ecommerce/control/main.
2. Select the "Giant Widget with feature explosion" from the Popular Categories section on home page. You will be redirected to product detail page.
3. Select color and size from the respective dropdowns and click on Add To Cart button.
4. Product is not getting added in cart.

Error on browser console:

getJs?name=ProductDetail:52 Uncaught TypeError: Cannot read property 'value' of undefined
 at addItem (getJs?name=ProductDetail:52)
 at <anonymous>:1:12
addItem @ getJs?name=ProductDetail:52
(anonymous) @ VM1370:1

Expected: Product should be added successfully.