Default Presets disappears from the drop-down menu in Preset Managers when resetting from Unified Player Settings.

Steps to repro:
1. Open a new project.
2. Create some Presets (From the light and Camera component in the scene for example)
3. Edit > Settings.
4. Select the Preset Manager from the left panel.
5. Add light and camera as default from the dropdown menu (+ at the bottom of the list)
6. Click on Cogwheel > Reset.

Actual Result:
The + menu is still empty which prevent you from adding back the Light or Camera

Expected result:
The + menu is populated back with all compatible Presets in the project.

Reproducible on:
2018.3.0a5, 2018.3.0a4.

Environment:
Occurring on Windows and Mac.