How to reproduce:
1. Open the attached user's 'Prefab bug.zip' project
2. Add "bridge_arch" fbx asset to the Scene
3. Right click on "bridge_arch" object in the Scene and select "Open Prefab Asset"

Expected result: Imported object can't be prefab
Actual result: Hierarchy view is offset when trying to open the FBX model in Prefab Mode

Reproducible: 2018.3.0a6, 2018.3.0b11, 2019.1.0a1
Fixed: 2018.3.0b12, 2019.1.0a3, 2019.1.0a10

Notes:
- The Prefab Mode was added in Unity 2018.3.0a6