During destruction toolbar_devices is nullified, but
gtk_widget_in_destruction() does not perform a NULL-check, and we could
crash:
==22708== Invalid read of size 1
==22708== at 0x505CC0B: gtk_widget_in_destruction (gtkwidget.c:10643)
==22708== by 0x189553: gcm_prefs_list_box_row_selected_cb (cc-color-panel.c:1708)
==22708== by 0x4A61714: g_cclosure_marshal_VOID__OBJECTv (gmarshal.c:1910)
==22708== by 0x4A5E148: _g_closure_invoke_va (gclosure.c:895)
==22708== by 0x4A784F3: g_signal_emit_valist (gsignal.c:3462)
==22708== by 0x4A78722: g_signal_emit (gsignal.c:3612)
==22708== by 0x4F5B121: gtk_list_box_remove (gtklistbox.c:2420)
==22708== by 0x4F5B222: gtk_list_box_dispose (gtklistbox.c:439)
==22708== by 0x4A63338: g_object_unref (gobject.c:3891)
==22708== by 0x4EAA994: gtk_box_dispose (gtkbox.c:230)
==22708== by 0x4A63338: g_object_unref (gobject.c:3891)
==22708== by 0x4EAA994: gtk_box_dispose (gtkbox.c:230)
==22708== Address 0xfffffffffffffeb2 is not stack'd, malloc'd or (recently) free'd
See: https://bugs.launchpad.net/ubuntu/+source/gnome-control-center/+bug/2009913
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