System Information
Operating system: Darwin-18.7.0-x86_64-i386-64bit 64 Bits
Graphics card: AMD Radeon Pro 580X OpenGL Engine ATI Technologies Inc. 4.1 ATI-2.11.20

Blender Version
Broken: version: 2.81 (sub 11), branch: master (modified), commit date: 2019-09-16 22:52, hash: rB76650402f300
Worked: (optional)

Short description of error
Blender crashes under certain conditions when a window is merged closed or otherwise deleted (quitting Blender, opening a new file).

Exact steps for others to reproduce the error
I am not sure how I got it into this state, but here is a simple blend file that exhibits the behavior. Try to merge the top two windows, or simply start a new file, and it will crash.

I tracked it down to a double free() in space_view3d.c in view3d_free(). It frees vd->shading.prop, but view3d_duplicate() just below does not make a copy of that as it does for fx_settings.dof and fx_settings.ssao. If a view is duplicated that has an attached shading property, that property will be freed twice when the the second window is closed.