MissingReferenceException on Scene Unload

Steps to reproduce:

1. See https://discussions.unity.com/t/surface-cache-gi-preview/1720494/405

Actual results: MissingReferenceException 

Expected results: no errors

Reproducible with versions:

Not reproducible with versions: 

Can’t test with versions: 

Tested on (OS): 

Notes:

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