WSA 8.1 and UWP applications crash when scene loaded asynchronously is unloaded.

Reproducible on 5.3.6p2, 5.4.0p1.

Repro steps:
1. open project "SceneLoadingSample.zip";
2. build to WSA 8.1 or UWP platforms;
3. build and run.
4. press button "load async". Press button Unload. Application crashes.

-- WSA 8.1 project crashes with error:
GetManagerFromContext: pointer to object of manager 'LightmapSettings' is NULL (table index 25)
(Filename: C:/buildslave/unity/build/Runtime/BaseClasses/ManagerContext.cpp Line: 89)

-- UWP project crashes with error:
GetManagerFromContext: pointer to object of manager 'RenderSettings' is NULL (table index 24)
(Filename: C:\buildslave\unity\build\Runtime/BaseClasses/ManagerContext.cpp Line: 89)