[FIX] web: invalid domain in DomainSelector

A domain could contain the key `parent` (in an embedded subview), like:

    domain="[('display_name', '=', parent.display_name)]")

This is currently not handled by the DomainSelector.

This will probably be handled in a future version but for this stable version,
this case that triggered an error is now catched and an error message is
displayed in the widget.

This was causing a traceback when opening a domain with `parent` in the edition
of a subview with Studio.