Assertion failed on expression: 'modifications.empty exception is thrown on creating prefab variant.

Steps to repro:
1. Open the attached project.
2. Select Cube prefab in the project view.
3. Right Click > Create > Prefab Variant.

Actual Result:
The exception is thrown in the console.

Expected Result:
Prefab variant creates.

Reproducible in:
2019.3.0a4, 2019.2.0b4.

Working Fine in:
2019.2.0b3.

Environment:
Windows & Mac.