Fix dangling pointer after scrolled_container destruction.

After the cnnobj->scrolled_container is automatically destroyed by GTK bacause of a parent desctruction, the cnnobj->scrolled_container pointer can be still point to the old destroyed widget. This fix allows automatic nullifying of cnnobj->scrolled_container pointer when the widget is destroyed.

Fixes #2734 (closed) Related to MR !2391 (merged)

/cc @PHWR FYI