Create prefab variant option is enabled even when no prefab is selected. It is always enabled by default when a new project is created.

Steps to reproduce:
1) Create a new project in improved prefabs build
2) Project view > Right click > Create > Prefab variant.
OR
Can also check from file menu Assets > Create > Prefab variant

Actual result: Prefab variant option is always enabled.

Expected result: Prefab variant option should only be enabled when a prefab is selected.

Reproducible on :
1)2018.2.0x,
2) 2018.3.0a2 (e9113db2e9bf)
Thu, 14 Jun 2018 21:56:37 GMT
Branch: prefabs/improved
Note :
- Nothing happens if a user selects prefab variant option if no prefab was selected.
- Prefab variant option is disabled by default on Mac and works as expected.

Environment: Only on windows