To reproduce:
1. open the attached project on 5.4.0f3
2. make sure unity_icon1 in the scene has Sprites-Default material on it
3. close the project
4. open the project with 5.5.0a5
5. observe how Sprites-Default material has disappeared from unity_icon1 and None material is shown instead

NOTE:
If project was created on 5.3 you can open it with both 5.4 and 5.5 without experiencing this issue.

Expected: When opening the project in 5.5 Sprite renderer material parameter should have Sprites-Default selected
Actual: When opening the project in 5.5 Sprite renderer material parameter has nothing selected

Reproduced with: 5.4.0f3 to 5.5.0a2, 5.4.0f3 to 5.5.0a5
Not reproducible: 5.3.4f1 to 5.5.0a5, 5.3.6p2 to 5.5.0a5