Subject: Duplicate a skeleton layer and delete a bone in the duplicated crash

Add skeleton, duplicate the layer, edit the duplicated and try to delete a
bone, it crashes.

CONSOLE OUTPUT
ss-debug: /home/carlos/Software/Synfig-git/install/include/ETL/_handle.h:280: T* etl::handle<T>::operator->() const [with T = synfig::ValueNode]: Afirmación `obj' fallida.
Cancelado

Git Revision: "New bug #6"
(Maybe related with the 000001)

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This also happens in the master branch with blines.

1. draw a bline outline
2. right-click it and 'duplicate'
3. select the duplicate (it's the top one in the layer list)
4. stretch the params dialog so it's about half the screen width and height
5. expand the vertices
6. scroll down so the vertices start towards the top of the params panel
7. remove (smart) the first vertex - it crashes

See sf.net bug #2412776.

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A recent commit mostly fixed the problem, but it still crashes like this:

1. draw a bline outline
2. encapsulate it
3. export the canvas parameter
4. open the new canvas in its own window
5. focus the new window
6. copy and paste the outline layer
7. stretch the params dialog so it's about half the screen width and height
8. expand the vertices
9. scroll down so the vertices start towards the top of the params panel
10. remove (smart) the first vertex - it crashes
