If Metal Editor Support* (Experimental) is enabled and Library folder is deleted, a project can no longer be opened. It freezes at loading bar.

Steps to reproduce:
1. Create new project
2. Import Spine Assets package (attached)
3. fix api upgrade errors
4. Enable Metal Editor Support* (Experimental) in Player Settings
4. Save and close Unity
5. Delete Library folder inside project files
6. Open the project

Result: Unity freezes at loading the project.

Editor.log output:
Updating Assets/Examples/Spine/Strechyman/stretchyman_SkeletonData.asset - GUID: 162719d41016c854abf0355feb0e14e8...
Metal: GrabIntoRenderTexture mismatched grab pass: 0 / 80
(Filename: /Users/builduser/buildslave/unity/build/Runtime/GfxDevice/metal/TextureReadbackMetal.mm Line: 143)
Receiving unhandled NULL exception
Obtained 12 stack frames.

Reproduced with: 5.6.1p2, 2017.1
5.5.3p3 (no Metal Editor Support* (Experimental) feature in this version).

Note: not reproducible on Windows.