Otherwise the weak reference is left dangling, which later causes issues at the time of destroying that widget. https://bugzilla.gnome.org/show_bug.cgi?id=726749
And let GtkPopover use it as its GtkAccessible implementation, this accessible sets the POPUP_FOR relationship to the relative-to widget, and keeps track of changes there. https://bugzilla.gnome.org/show_bug.cgi?id=725864