Not sure what to tell you.
I just re-downloaded the SelectionWidget v1.5, added to a new FlexViewer3 project (uncompiled version), added to my module list and let it build.
I do get warning about the dispatchEvent, but if I ignore it, and do NOT make any changes (as I did above)(it seems the changes remove a warning, but don't affect functionality), it seems to be working just fine. even with the SelectionWidget.xml that came with it, though I am not loading any of the services selected in my map.
Once I change the config to my services, it is still working fine.
Have not seen the error you mention and can't seem to reproduce.
R_
I am using IE8, Flash Builder 4.6 and the 4.6.0 SDK