This error seems to only happen in a combination of Flash 8 run on older versions of Mac OS X.
The only workaround at the moment is to disable "Listen for Stage Updates" checkbox in Smart Magnet Rig panel and keep it unchecked at all times.
Panel can still be used for creating the hierarchies and assigning metadata to symbols on stage; only some of the visual feedback will be missing.
When rigging is complete it will be best to keep panel closed.