TBS fails silently on gradient marker click

Starting the painting tutorial, I resaved Painting_Rough.tbp under a new name, went to the Color tab on Properties, duplicated the orange color swatch, double clicked the new swatch, selected “gradient” and “radial”. All went well so far. Then when I clicked the gradient marker below the gradient bar, TBS just closed with no error message. I tried again. Same thing. I rebooted and tried again. Same thing.

Has anyone else experienced this, or anything similar? Any idea what might be causing it?

Thanks,
Mike Hurwicz