When "Insert Base" is selected from the confirmation dialog while creating Prefab of a Prefab variant and then opening it asks to Save the Modified Prefab, saving the prefab variant causes editor to throw errors

Steps to reproduce:
1. Create a new project > Create a Cube game object.
2. Create prefab.
3. Create a Prefab Variant of this prefab
4. Open the created Prefab Variant in Prefab Mode
5. Make sure 'Auto Save' is toggled off (in the the Scene View upper right corner)
5. Drag 'Cube Variant' root Prefab instance to Project Browser
6. Choose 'Insert Base' option in Change base confirmation dialog
7. Select Cube Variant Prefab in Project Browser > save.

Actual result:
"ArgumentException" error thrown in console

Expected result:
No error in console

Reproducible in:
2020.2.0a12, 2020.2.0a19, 2020.2.0b2

Working fine on:
2020.2.0a11, 2020.1.0f1, 2019.4.5f1

Environment:
Occurring on Windows 10 & macOS 10.15