How to reproduce:
1. Open the attached ARTest.zip project
2. Go to Menu bar-> Edit-> Project Settings-> XR-> ARKit set the Requirement to Optional
3. Build for iOS and deploy the Xcode project to the iOS device
===Crash occurs===

Reproducible with: 2018.4.6f1, 2019.1.13f1, 2019.2.0f1, 2019.3.0a11

Reproducible with:
iPhone 7 (iOS 10.2)
iPad 5th gen (iOS 10.3.3)

Note: Go to linking settings Build Phases > Link binary with libraries and inspect that UnityARKit.a status is "Required" instead of "Optional"
Setting UnityARKit.a status manually to "Optional" via Xcode results with liker error

This setting was introduced in ARKit XR Plugin preview.3-2.1.0

Stack trace:

• thread #1, stop reason = signal SIGABRT

frame #0: 0x0000000105d868bc dyld`__abort_with_payload + 8
frame #1: 0x0000000105d86268 dyld`abort_with_payload_wrapper_internal + 100
frame #2: 0x0000000105d86294 dyld`abort_with_payload + 12
frame #3: 0x0000000105d643bc dyld`dyld::halt(char const) + 316
frame #4: 0x0000000105d669c4 dyld`dyld::_main(macho_header const, unsigned long, int, char const**, char const**, char const*, unsigned long) + 4416
frame #5: 0x0000000105d61044 dyld`_dyld_start + 68