We not only need to make sure the iterator remains valid, but also
the object pointed to.
Valgrind trace:
Invalid read of size 8
Gtkmm2ext::Pane::forall_vfunc(int, void (*)(_GtkWidget*, void*), void*) (pane.cc:617)
Gtk::Container_Class::forall_vfunc_callback(_GtkContainer*, int, void (*)(_GtkWidget*, void*), void*)
gtk_container_destroy (gtkcontainer.c:1073)
g_closure_invoke (gclosure.c:804)
...
g_object_run_dispose (gobject.c:1084)
