[LWRP] In project view previews don't have their materials after importing and upgrading new assets

To reproduce:
1. Create a new project
2. Install Lightweight pipeline
3. Import Low Poly Rocks Pack asset from the asset store
4. Edit -> Render Pipeline -> Upgrade Project Materials to LightweightRP Materials
5. Locate some of the rock prefabs in the project (Poly Rocks Pack/Rock Assets/Prefabs/With_Box_Colliders/2 Color/Round_Rocks for example)
Expected result: You can see their thumbnails correctly
Actual result: Material is not properly loaded, resulting in a magenta outline.
Note: You can select any of these and the preview works normally.

Reproduced on:
2019.2.0a12 with 6.6.1 LightweightRP
2018.1.9f2 with 1.1.11-preview