To repro:
1. Create tests in TestRunner.
2. Open tests in Rider.
3. Set a breakpoint.
4. Attach Rider to Unity Editor in Debug mode.
5. In Editor confirm switching to Debug mode.
6. Doubleclick on tests in TestRunner (it may take several reruns to crash the editor).

Actual result: Editor either crashes or freezes.

Errors in Rider:
Debug symbols for assembly nunit.framework could not be loaded correctly. Mono runtime doesn't support full pdb

Stacktrace:

31 ??? 0x00000001492548d9 0 + 5522147545
32 libmonobdwgc-2.0.dylib 0x00000001418969fc mono_jit_runtime_invoke + 2519
33 libmonobdwgc-2.0.dylib 0x0000000141a59035 do_runtime_invoke + 80
34 libmonobdwgc-2.0.dylib 0x0000000141a58f93 mono_runtime_invoke + 31
35 com.unity3d.UnityEditor5.x 0x0000000106249717 scripting_method_invoke(ScriptingMethodPtr, ScriptingObjectPtr, ScriptingArguments&, ScriptingExceptionPtr*, bool) + 71
36 com.unity3d.UnityEditor5.x 0x00000001062430e7 ScriptingInvocation::Invoke(ScriptingExceptionPtr*, bool) + 471
37 com.unity3d.UnityEditor5.x 0x00000001052b54b9 BurstCompilerService::CompileAsync(ScriptingMethodPtr, void*, int, void (*)(void*, int, void*), void (*)(void*, BurstLogType, char const*, char const*, int), char const*) + 569
38 com.unity3d.UnityEditor5.x 0x00000001052b5267 BurstCompilerService::CompileAsync(ScriptingObjectPtr, void*, int, void (*)(void*, int, void*), void (*)(void*, BurstLogType, char const*, char const*, int), char const*) + 263
39 com.unity3d.UnityEditor5.x 0x00000001052b58c8 BurstCompilerService::CompileAsync(ScriptingClassPtr, ScriptingObjectPtr, void*, int, void (*)(void*, int, void*), void (*)(void*, BurstLogType, char const*, char const*, int)) + 616
40 com.unity3d.UnityEditor5.x 0x0000000105939664 CreateJobReflectionData(ScriptingSystemTypeObjectPtr, ScriptingSystemTypeObjectPtr, JobType, ScriptingObjectPtr*, core::basic_string<char, core::StringStorageDefault<char> >&) + 3204
41 com.unity3d.UnityEditor5.x 0x000000010593b8bd CreateJobReflectionData(ScriptingSystemTypeObjectPtr, ScriptingSystemTypeObjectPtr, JobType, ScriptingObjectPtr, ScriptingObjectPtr, ScriptingObjectPtr) + 109
42 com.unity3d.UnityEditor5.x 0x0000000106407193 JobsUtility_CUSTOM_CreateJobReflectionData(ScriptingBackendNativeObjectPtrOpaque*, ScriptingBackendNativeObjectPtrOpaque*, JobType, ScriptingBackendNativeObjectPtrOpaque*, ScriptingBackendNativeObjectPtrOpaque*, ScriptingBackendNativeObjectPtrOpaque*) + 275
43 ??? 0x000000017220036c 0 + 6209667948

Expected result: If we're not able to debug tests, it has to be appropriately handled (clear warnings in Console, no crashes, etc).