Crash separating shared vanishing points (sometimes)

Steps to reproduce:

1. open Inkscape
2. create 3 or 4 3D boxes
3. select the 2nd one you created
4. Shift+Click on the left-most vanishing point to move it
5. close document and repeat steps 2-4 (usually only need to repeat once)

What happened?

• crash, if debug build used, some asserts and g_warnings are raised (prepended to attached backtrace): inkscape-backtrace-master-3dbox.txt

What should have happened?

• no crash

Version Info:

• Inkscape 1.1-dev (9d9d1f1dcb, 2020-11-03) debug build Linux Mint 20
• Inkscape 1.1-dev (4fbe0dc968, 2020-11-01) appimage Linux Mint 20
• Inkscape 1.0.1 (0767f8302a, 2020-10-17) snap Linux Mint 20
• Inkscape 1.0alpha (9dee831, 2019-01-15) appimage Linux Mint 20
• Inkscape 0.92.4 (cbf31a1a41, 2019-01-14) debug build Linux Mint 20
• Inkscape 0.92.2 (5c3e80d386, 2017-08-06) glad I got it built somehow, Linux Mint 20