How to reproduce:
1. Create a new Unity project
2. Import the Unity NetCode package
3. Create a Cube and turn it into a Prefab
4. Create a Prefab Variant from the Cube Prefab Instance
5. Add a GhostAuthoringComponent and a ConvertToClientServerEntity Components to the Variant
6. Click 'Update Component List' on the GhostAuthoringComponent

Expected result: Prefab Variants are supported
Actual result: InvalidOperationException is thrown

Reproducible with: 2019.3.13f1, 2020.1.0b8, 2020.2.0a10
Could not test with: 2017.4.40f1, 2018.4.22f1(package unavailable)

Reproducible with package versions: preview.6 - 0.1.0
Not reproducible with package versions: preview.0 - 0.0.4