OS X El Captain 10.11.6
AMD FirePro D500 3072 MB
(example: 2.79b release)
(example: 2.80, edbf15d3c044, blender2.8, 2018-11-28, as found on the splash screen)
Short description of error
When I try to use Auto Mirror in this file I get the error message in the screen grab (attached).
I might be doing something obviously wrong.
Never had that before though
It works ok in different files.
Exact steps for others to reproduce the error
Based on the default startup or an attached .blend file (as simple as possible).