Editor crash while assigning multimaterial to spline-aligned meshes

Here’s the stack from the crash. I was creating a spline aligning power poles along the road, about 1.5 km long, with meshes repeating every 30 meters. When I assigned the material to the spline, the editor crashed producing this error:

First-chance exception at 0x00000000696B73F0 (Qt5Core.dll) in Editor.exe: 0xC0000005: Access violation reading location 0x0000000000000000.
Unhandled exception at 0x00000000696B73F0 (Qt5Core.dll) in Editor.exe: 0xC0000005: Access violation reading location 0x0000000000000000.