How to reproduce:
1. Open the attached project named "Case_1231777"
2. Select the "hammer_low.fbx" in the Inspector
3. Observe that the model is visible in the Preview window
4. Select the Forward renderer asset
5. Uncheck any one of the Opaque Layer Mask options or set it to "Nothing"
6. Observe the hammer_low.fbx in the Preview window again

Expected result: The model is visible in the Preview window
Actual result: Nothing is rendered in the Preview window

Reproducible with: 2019.3.8f1, 2020.1.0b4, 2020.2.0a5
Could not test with: 2017.4.38f1(No package manager), 2018.4.20f1(Forward renderer asset lacked the Opaque Layer Mask options)

Reproducible with package versions: 7.3.1, 8.0.1, 9.0.0-preview.8

Workaround: The bug can be solved by creating a layer at index 31 and make sure this is enabled in for the opaque layer mask on the forward renderer.

Notes:
- Opaque Layer Mask options became available in URP 7.3.1