Reproduction steps:
1. Open the attached project "1221566.zip"
2. Open the Sample Scene
3. Select Sub Scene in the Hierarchy
4. Press Edit in the Inspector
5. Enter Play mode

Expected result: No errors are thrown and the imported GameObject falls on the Plane
Actual result: InvalidOperationException is thrown and the imported GameObject falls through the Plane

Reproducible with: 2019.3.3f1 (Entities 0.2.0-preview.18, 0.6.0-preview.24), 2020.1.0a24 (Entities 0.6.0-preview.24)

Notes:
Enabling Read/Write in the GameObject import settings will prevent the issue.
The issue is reproducible only when the subscene is being edited.