How to reproduce:
1. Open the attached "1252763 repro" project
2. Open the "SampleScene" from the Assets folder
3. Open the Addressables Groups window from Window > Asset Management > Addressables > Groups
4. Go to Packages > PrefabTest > Editor folder
5. Select the "GameObject" from the Hierarchy window
6. Select the "Prefab" from the Project window

Expected results: no errors are generated
Actual results: "ArgumentException: Can't save an immutable Prefab" errors are continuously generated in the Console window

Reproducible with: 0.0.8-preview (2018.4.23f1), 0.0.27-preview (2018.4.23f1), 1.1.10 (2019.3.16f1), 1.2.4 (2019.3.16f1), 1.8.3 (2019.4.0f1), 1.8.4 (2020.1.0b11), 1.9.2 (2018.4.23f1, 2020.2.0a14)

Notes:
-The "GameObject" from the Hierarchy window which has a reference to the "Prefab" must be selected first in order to reproduce the bug
-After the bug has been reproduced, the issue persists even if the "Test Script" and the GameObject holding it are deleted.
-It only goes away if the Editor is restarted or the "Prefab" is reimported