Delete option is disabled for "Prefab Variant" when Root Prefab is deleted

Before going through the steps to reproduce apply the patch for the prefab importer.

Steps to repro:
1. Create a new project.
2. Hierarchy > Create Cube.
3. Drag Cube to Project Browser > make prefab
4. Right-click on 'Cube' Prefab Asset in Project Browser
5. Create -> Prefab Variant
6. Delete Root Cube Prefab
7. Try to delete Cube Prefab Variant

Actual Result:
Delete option is disabled when right-clicked on Prefab Variant

Expected Result:
Prefab Variant should be able to delete even after Root Prefab is deleted

Note: Refer attached video

Reproducible in:
2019.3.0a12

Working Fine in:
2019.3.0a11, 2019.2.1f1.

Environment:
Windows 10 and macOS 10.14

Workarounds:
1. Prefab Variant can be deleted from Project Explorer
2. Variant asset can be deleted after Reimport (through right-click menu on the asset)