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.