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.
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I am having a similar problem with random crashes and no errors. Program just disappears, often when I move the skeleton while in animation mode. It does not do it all the time but about 2 to 3 times per hour.
(All updates and changes below have included rebooting the computer.)
- Windows 10 (updated fully)
- NVidia GEForce RTX 2060 Super (updated fully)
- Dual monitor (both the same and using the same settings)
- Synfig Studio 1.5.1 64-bit (the previous stable version crashed the same way)
- I have tried deleting the C:\Users\MyName\Synfig\ files
- There doesn’t seem to be an exact sequence of events to make it crash. The last crash happened when I rolled the mouse button to move the canvas while preparing to move the skeleton.
FOLLOWUP: I uninstalled Synfig 1.5.1 completely. Rebooted the computer then reinstalled again. The problem seems to have not come back.
Synfig still crashes with the color picker which I posted about here.