Operating system: Darwin-17.7.0-x86_64-i386-64bit 64 Bits
Graphics card: AMD Radeon RX 560 OpenGL Engine ATI Technologies Inc. 4.1 ATI-1.68.25
Broken: version: 2.90.0, branch: master, commit date: 2020-08-31 11:26, hash: rB0330d1af29c0
(Broken too: version: 2.91.0 alpha, commit date: 2020-08-31 22:30, hash: '4e06afb0a1d6')
Worked: 2.83.5 LTS (and all previous 2.8x ones. I think some previous 2.90 beta did, too, but I can't be certain, sorry)
Short description of error
Blender crashes when the renderer is set to Cycles and:
a) The viewport is then switched to render preview or
b) A new material is created.
Exact steps for others to reproduce the error
a) On a default startup project, go to the Properties panel > Render properties tab, and switch the Render Engine from Eevee to Cycles. Then click on a 3D viewport's Display Render Preview widget. Blender should crash immediately.
b) On a default startup project, go to the Properties panel > Render properties tab, and switch the Render Engine from Eevee to Cycles. Then go to the Material Properties tab, delete the default cube's material, then press the Add a new material button. Blender should crash immediately.