Fix chain select bug when using the same widget template twice on the same page

Steps to reproduce:

• display a layout with two distinct widgets both using the "coverage_widget.xhtml" template
• submit the form

=> NullPointerException in ChainSelect

This is due to bad referencing between chain select sub components: trying to fix the error by using safer code will result in a validation error as the second chain select will not resolve the second select value correctly (and incomplete selections are not allowed)