How to reproduce:
1. Open the attached 'TestUnityProject.zip' project
2. Open 'PostProcess' Script in Rider Code Editor
3. Add breakpoint in 'string result = report.summary.outputPath;' variable
4. Attach Rider Debugger to the opened project
5. Navigate File-> Build Settings-> Switch to iOS Platform
6. Build for iOS

Reproducible with: 2019.3.0a8(1.0.8), 2019.4.0f1(1.1.4), 2020.1.0b11(1.2.1), 2020.2.0a13(2.0.3)
Not reproducible with: 2018.4.23f1, 2019.3.0a7(1.0.8)

First 5 lines of the stacktrace:
#0 0x007fff67bd32c6 in __pthread_kill
#1 0x007fff67b3d6a6 in abort
#2 0x000001485b2ab2 in mono_log_write_logfile
#3 0x000001485c6abc in monoeg_g_logv
#4 0x000001485c6c61 in monoeg_assertion_message
#5 0x0000014852f891 in mono_metadata_decode_row

Notes:
- Could not reproduce with VSCode, Visual Studio
- Reproducible with Mac Standalone build (inconsistent); Not reproducible with Windows Standalone build;
- Tested with Rider 2020.1.3
- Video of reproduction is attached in Edit