Steps to reproduce:

1) Download attached project and open in Unity
2) Open scene "SampleScene" and make sure that Post Process Layer(Script) component is on the MainCamera gameObject
3) Go to Player Settings and make sure that graphics API is set to Metal
4) Build and Run "SampleScene" on the device

Note: Application will crash with the following stacktrace:
2018-04-20 11:43:04.497 UnityStrippedReproCrashiOS[570:245630] -[MTLDebugFunction setLabel:]: unrecognized selector sent to instance 0x14120f0d0
2018-04-20 11:43:04.602 UnityStrippedReproCrashiOS[570:245630] Uncaught exception: NSInvalidArgumentException: -[MTLDebugFunction setLabel:]: unrecognized selector sent to instance 0x14120f0d0
(
0 CoreFoundation 0x0000000184086dc8 <redacted> + 148
1 libobjc.A.dylib 0x00000001836ebf80 objc_exception_throw + 56
2 CoreFoundation 0x000000018408dc4c <redacted> + 0
3 CoreFoundation 0x000000018408abec <redacted> + 872
4 CoreFoundation 0x0000000183f88c5c _CF_forwarding_prep_0 + 92
5 UnityStrippedReproCrashiOS 0x000000010061c0d8 _ZN14GfxDeviceMetal20CreateComputeProgramEPKhmPKc + 228
6 UnityStrippedReproCrashiOS 0x0000000100e77e8c _ZN15GfxDeviceWorker10RunCommandER20ThreadedStreamBuffer + 40320
7 UnityStrippedReproCrashiOS 0x000000010061d464 GfxDeviceWorkerAutoreleasePoolProxy + 68
8 UnityStrippedReproCrashiOS 0x0000000100e785ac _ZN15GfxDeviceWorker6RunExtER20ThreadedStreamBuffer + 144
9 UnityStrippedReproCrashiOS 0x0000000100e78510 _ZN15GfxDeviceWorker3RunEv + 148
10 UnityStrippedReproCrashiOS 0x0000000100e6df7c _ZN15GfxDeviceWorker18RunGfxDeviceWorkerEPv + 12
11 UnityStrippedReproCrashiOS 0x0000000100a304d8 _ZN6Thread16RunThreadWrapperEPv + 64
12 libsystem_pthread.dylib 0x0000000183cebb28 <redacted> + 156
13 libsystem_pthread.dylib 0x0000000183ceba8c <redacted> + 0
14 libsystem_pthread.dylib 0x0000000183ce9028 thread_start + 4
)
2018-04-20 11:43:04.603 UnityStrippedReproCrashiOS[570:245630] *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[MTLDebugFunction setLabel:]: unrecognized selector sent to instance 0x14120f0d0'
*** First throw call stack:
(0x184086db0 0x1836ebf80 0x18408dc4c 0x18408abec 0x183f88c5c 0x10061c0d8 0x100e77e8c 0x10061d464 0x100e785ac 0x100e78510 0x100e6df7c 0x100a304d8 0x183cebb28 0x183ceba8c 0x183ce9028)
libc++abi.dylib: terminating with uncaught exception of type NSException
(lldb)

Reproduced with:
2018.2.0a9, 2018.2.0b1

Not reproduced with:
2018.1.0f1, 2018.2.0a8

Devices under testing:
Reproduced with:
000037 iPad Mini 2, iOS 9.3.4
000001 iPhone 6+, iOS 8.0

Not reproduced with:
000366 iPhone 7, iOS version: 11.2.6
VLNQA00124 Google Pixel 2 XL*, OS:8.1.0, CPU:arm64-v8a, GPU:Adreno (TM) 540
VLNQA00128 Samsung SM N950F*, OS:8.0.0, CPU:arm64-v8a, GPU:Mali-G71

Not reproduced using Gles3 Graphics API

Post Process stack was downloaded using PackageManager. Version of the Post Processing stack : 2.0.3