Reproduction steps:
1. Open the attached project ("1323716RE.zip")
2. Open the "TutorialScene" scene
3. Open Rider editor and attach to Unity process
4. Set a breakpoint on Packages/com.unity.visual-compositor/Runtime/Timeline/VisualCompositorBehaviour.cs at line 18
5. Start debug process
6. Open Timeline window (Window -> Sequencing)
7. Select Timeline Game object
-- Observe the crash

Reproducible with: 2019.4.24f1, 2020.2.7f1, 2020.3.2f1, 2021.1.0f1, 2021.2.0a12
Could not test with: 2018.4.32f1 (multiple nunit, package errors)

Top 5 lines of the stack trace:
0x00007FFFBD3A5748 (mono-2.0-bdwgc) \[c:\\build\\output\\unity-technologies\\mono\\mono\\mini\\debugger-agent.c:7686] buffer_add_value_full
0x00007FFFBD3B0869 (mono-2.0-bdwgc) \[c:\\build\\output\\unity-technologies\\mono\\mono\\mini\\debugger-agent.c:11852] object_commands
0x00007FFFBD3A7973 (mono-2.0-bdwgc) \[c:\\build\\output\\unity-technologies\\mono\\mono\\mini\\debugger-agent.c:12398] debugger_thread
0x00007FFFBD2567F8 (mono-2.0-bdwgc) \[c:\\build\\output\\unity-technologies\\mono\\mono\\metadata\\threads.c:1043] start_wrapper_internal
0x00007FFFBD256586 (mono-2.0-bdwgc) \[c:\\build\\output\\unity-technologies\\mono\\mono\\metadata\\threads.c:1102] start_wrapper
<...>