PIE gets stuck on infinite loop when Background Streaming is disabled

When "Use Background Level Streaming" is disabled, the Editor gets stuck loading in PIE when you press Play. This is a regression from 4.19.2 (CL-4033788).

This issue was reported and tested in 4.20.2 (CL-4302132). This was reproduced in Main 4.21 (CL-4313833)

1. Download and open the attached project
2. Click on Edit and go to Project Settings
3. Search for "Use Background Level Streaming" and disable it if it is enabled
4. Play in Editor

Result: The Editor gets stuck trying to load PIE

Expected: PIE loads up and you can move around