Reproduction steps:
1. Open the attached project.
2. Open and play the LoadTest scene.

Actual behavior:
- The editor will freeze/crash.

Expected behavior:
- The editor will show an error message and moves along.

Reproduced with:
Editor, iOS
Metal
2017.3.0p2, 2018.1.0b2

Not reproducible with:
OpenGL, OpenGLES
or
2017.2.1p2

Editor Metal freeze:
WARNING: PVRTC texture format is not supported, decompressing texture
Receiving unhandled NULL exception
Obtained 10 stack frames.

iOS Metal crash:
* thread #15, name = 'UnityGfxDeviceWorker', stop reason = EXC_BAD_ACCESS (code=1, address=0x0)
* frame #0: 0x0000000100358a28 ProductName`::CreateMetalShaderShaderLab() [inlined] ExtractMetalShaderBlobDesc at MetalShaderBlobHeader.h:39 [opt]
frame #1: 0x0000000100358a28 ProductName`::CreateMetalShaderShaderLab() at GpuProgramsMetal.mm:205 [opt]
frame #2: 0x0000000100529780 ProductName`::CreateGpuProgram() at GpuProgram.cpp:883 [opt]
frame #3: 0x00000001005f317c ProductName`::DequeueAll() at GfxCreateGpuProgramQueue.cpp:33 [opt]
frame #4: 0x000000010060a68c ProductName`::RunCommand() at GfxDeviceWorker.cpp:606 [opt]
frame #5: 0x0000000100354dbc ProductName`::GfxDeviceWorkerAutoreleasePoolProxy() at GfxDeviceMetal.mm:3592 [opt]
frame #6: 0x000000010060c040 ProductName`::RunExt() [inlined] ProcessGfxCommands at GfxDeviceWorker.cpp:143 [opt]
frame #7: 0x000000010060c018 ProductName`::RunExt() at GfxDeviceWorker.cpp:345 [opt]
frame #8: 0x0000000100604314 ProductName`::RunGfxDeviceWorker() at GfxDeviceWorker.cpp:310 [opt]
frame #9: 0x00000001004f8c9c ProductName`::RunThreadWrapper() at Thread.cpp:36 [opt]

OpenGL error message:
WARNING: 281176480 format is not supported, decompressing texture
WARNING: Shader Unsupported: 'Twnkls/MobilePB/OpaquePB' - Pass '' has no vertex shader
WARNING: Shader Unsupported: 'Twnkls/MobilePB/OpaquePB' - Pass '' has no vertex shader
WARNING: Shader Unsupported: 'Twnkls/MobilePB/OpaquePB' - All passes removed
WARNING: Shader Unsupported: 'Twnkls/MobilePB/OpaquePB' - Setting to default shader.
WARNING: Shader Unsupported: 'Twnkls/MobilePB/Shadow' - Pass '' has no vertex shader
WARNING: Shader Unsupported: 'Twnkls/MobilePB/Shadow' - Setting to default shader.