wizard: don't work with destroyed widgets

Resolves rhbz#1133055
Related to rhbz#1069917

Signed-off-by: Jakub Filak <jfilak@redhat.com>
wizard: "Next" button insensitive on the review page by default

This feature was accidentally removed in commit
b4044bad5c86b552e252376f1b520afa28c8c63e

Signed-off-by: Jakub Filak <jfilak@redhat.com>