Editor freezes while saving the scene if materials have default type texture assigned to normal maps in import settings.

Steps to repro:
1. Open the attached project.
2. Select "Test" in the project window.
3. Inspector > Normal Map > Assign "Test" texture.
4. Save the scene.

Actual Result:
Editor freezes while saving the scene.

Expected Result:
The editor should not freeze.

Reproducible in:
2019.3.0a7, 2019.3.0a1.

Working fine in:
2019.3.0b7.

Environment:
Windows & Mac.