Steps to reproduce:
1. Open attached project "Mech_Builder.zip"
2. Open SampleScene
3. Select "Camera" in the Hierarchy
4. Set "Clear flags" to "Depth Only"

Actual results: Editor stops working (goes blank)

Reproduced in: 2018.3.0b12, 2018.3.6f1,2019.1.0b3, 2019.2.0a5

Edit:
1. The bug only reproduces if setting from"Skybox" to "Depth Only"
2. Could not reproduce with 2017,4, because the project uses improved Prefabs and one prefab "Mech_Humanoid_Lvl2_V2" is crucial to this bug.
3. Removing Camera "Target texture" also fixes this problem.