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

changed milestone to %v1.4.27

added bug label

assigned to @antenore

merged

mentioned in commit 0cbb0619

With the very last version (after a flatpak update), the "auto-scroll" when the mouse is moved to the screen border is not working anymore. Not very clear if this is related to this particular change.

See #2529 (closed)

mentioned in merge request !2414 (merged)