Assertion failed errors occur when painting speed trees in prefab editor when the light is being baked

Steps to Repro:
1. Create a new or open attached project
2. Hierarchy > right click
3. 3D Object > terrain
4. drag and drop terrain in the project window to create a prefab
5. Select the prefab and click "Open Prefab" button
6. Select 'Paint Trees' tab > edit tree
7. Add a speed tree
8. Window > rendering > lighting
9. Enable auto-generate and paint trees on the terrain

Actual Result:
1. Assertion failed error spams console
2. Turning off auto-generate lighting stops the errors

Environment:
Windows and Mac

Occurring with:
2018.3.0b1, 2018.3.0a6

Note:
Prefab Editor was introduced in 2018.3.0a6

Stack trace:
Assertion failed on expression: 'probeSetData.identifier.IsValid()'