How to reproduce:
1. Open the attached project "Case_1231721" and load scene "2b1. Motion Properties - Mass"
2. In the Hierarchy select "GameObject"
3. In the Inspector set Simulation type of Component "Physics Step" to either Havok Physics, or Unity Physics
4. Press Play

Expected result: Editor works normally
Actual result: Editor freezes, if Unity Physics was selected, or the Editor crashes if Havok Physics was selected

Reproducible with:
Havok Physics: 0.1.1-preview, 0.1.2-preview, 0.2.2-preview (2019.3.13f1, 2020.1.0b9, 2020.2.0a10)
Unity Physics: 0.1.0-preview (2019.3.13f1, 2020.1.0b9, 2020.2.0a10), 0.3.2-preview (2020.2.0a10)
Not reproducible with:
Unity Physics: 0.0.2-preview.1 (2019.3.13f1, 2020.1.0b9)
Could not test with:
0.0.2-preview.1 (2020.2.0a10, package throws errors), 2017.4.40f1, 2018.4.22f1 (Unity Physics and Havok Physics not available)

First lines of the Stack trace with Havok Physics selected:
0x00007FFC10CF6AC6 (HavokNative) HP_UnlockPlugin
0x00007FFC10D1BB01 (HavokNative) HP_UnlockPlugin
0x00007FFC10D05748 (HavokNative) HP_UnlockPlugin
0x00007FFC10B8DB67 (HavokNative) HP_UnlockPlugin
0x00007FFC10B02AB2 (HavokNative) HP_ProcessStep
0x00007FFC3F3810EA (ebf394a426309c84c4ccf28e43dfa06) 92443126A0D34A67
0x00007FF7B5B18FE4 (Unity) ExecuteJob
0x00007FF7B5B1924C (Unity) ExecuteJobCopyData

Notes:
1. Trying to change the Solver Iteration Count in Project Preferences above 255, caps it at 255