First i used the width tool on my skeleton then it suddenly crashes when i did so.
I recently fixed it, but just for not crashing.
Anyway, it is not supposed to use Width Tool for bones - although it could make sense, it is focused on change outline width.
You can change the bone tip/root width manually: select the skeleton layer, enable Width handles and drag them. Basic Bone Morphing — Synfig User Manual 1.5.1 documentation
The fix will be available for future releases.