Some sprite asset references are missing after upgrading a project to 2019.1.0a10 (and occasionally a9).

Reproduction Steps:
1. Open the attached project in 2019.1.0a9+
2. Observe that many sprites are missing from many game objects

Note that if opening the project in 2019.1.0a8, this isn't an issue, and opening the project in a9 after it's been opened in a8 doesn't show the issue either.. however then proceeding to open it in 2019.1.0a10 always demonsrates it.

Reproducible in versions 2019.1.0a9, 2019.1.0a10.
Not reproducible in versions 2018.3.0b11, 2019.1.0a8.