How to reproduce:
1. Download the project from the link provided in the Edit section
2. Go to the Project folder-> Packages-> Vuforia Engine AR-> Vuforia-> Prefabs
3. DoubleClick on any of the Prefabs to open them
4. Inspect the Console window

Actual: Very difficult to exit prefab editing mode and errors are spammed in the console (see the video attached)

The error spammed:
ArgumentException: Saving Prefab to immutable folder is not allowed: 'Packages/com.ptc.vuforia.engine/Vuforia/Prefabs/DefaultMidAirIndicator.prefab'

Reproducible with:
8.5.9 Vuforia Engine AR
8.5.8 Vuforia Engine AR
8.3.9 Vuforia Engine AR
8.1.12 Vuforia Engine AR

Reproducible with: 2018.4.12f1, 2020.1.0a11

Notes:
- The buttons like Scenes in the Scene window or Back button "<" are not responsive
- Errors are being spammed
- The editor is not closable and Force quite need to be used
- This issue appears with Vuforia package