Reproduction steps:
1. Open the attached project in "New Unity Project.zip"
2. The project uses Advertisements Package
3. Update Advertisements package to 3.4.7
4. Build the project and deploy it to an iOS device

Unexpected behavior: Application crashes just before the splash logo disappears (stack trace in edits)

Reproducible with Advertisements 3.4.7 (2020.2.0a16)
Not reproducible with Advertisements 3.4.5 (2020.2.0a16)

Devices tested:
VLNQA00310 iPad Pro 12.9 (iOS 13.4.1)
VLNQA00015, Samsung Galaxy Note8 (SM-N950W), Android 8.0.0, CPU: Snapdragon 835 MSM8998, GPU: Adreno (TM) 540

Few lines from trace:

• thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0xba)

frame #0: 0x000000010390857c UnityFramework`UnityAdsSettings_Get_Custom_PropTestMode() + 12
frame #1: 0x0000000103fc04b0 UnityFramework`Advertisement_Initialize_m6911885226E8E778314C6C1FFFBBDB98A1A6AD56 + 220
...

• frame #26: 0x0000000102103e1c NewUnityProject`main(argc=<unavailable>, argv=<unavailable>) at main.mm:26:9 [opt]

frame #27: 0x0000000183c77800 libdyld.dylib`start + 4