Sorting layer is shown as "Unknown Layer" after upgrading the project to 2018.3

Steps to Repro:
1. Download "2D Platformer" from asset store or open attached project
2. Select "Assets\Prefabs\Characters\enemy2"
3. Check it's sorting layer

Actual Result:
Sorting layer is unassigned for the enemy, as a result, game objects are not being rendered on playing the scene

Expected Result:
Sorting layers should be assigned on importing the package from the asset store and on upgrading the project from older unity version

Environment:
Windows and Mac

Occurring with:
2018.3.0a7, 2018.3.0a1

Working with:
2018.2.0f2