unwrap breaks under some circumstances

[Philippe Manzano]

I just had the exact same error as #68,
At the end, it throws Cannot normalize a vector of length 0.0
... but I found that it happens under a specific process: ...->unwrap->modify->unwrap

On Ubuntu 21.10, FreeCAD FreeCAD_0.19.3-Linux-Conda_glibc2.12-x86_64.AppImage
To be clear:
I add the default glider, unwrap it, it works.
Then I modify it and try to unwrap, it fails.
From there, if I create a new document and add a new glider and unwrap, it fails.
If I quit and relaunch FreeCAD, add default glider, MODIFY it and unwrap it works.
If I modify it again and unwrap it fails.
If I now save the (failing) glider, quit and relaunch FreeCAD, open the saved glider and unwrap it works ...

So unwrap might write something somewhere at first unwrap that is read at second unwrap, if you modify something in between unwraps, it fails.
Only saving, quitting FreeCAD and reopening to unwrap again works (it basically can't read the previous unwrap var somewhere).
But you know you can't unwrap anymore after this one, you'd better be sure you want to unwrap it :)