These materials were missing from the "Change Active Material" menu.
Caused by rBe3faef686d38.
Error was getting the preview [which wasnt there yet]
These only appeared once the material tab in the Properties Editor was
used (since this ensured a valid preview icon).
Above commit changed behavior for RNA icon getter (this does not create
data anymore), so ensure the preview by hand here.
Similar to rB182edd4c35c2.
Fixes T102566.
Maniphest Tasks: T102566
Differential Revision: https://developer.blender.org/D16541
3e81d177ea