Setting custom bake inputs has this check
```if (!GetProgressiveBakingBackendEnabled())
ErrorString("Custom bake inputs will not be processed. Please make sure to enable Baked Global Illumination and select Progressive as the Lightmapper before generating lighting.")```
and that spits out this error if the custom bake inputs are set in the `Lightmapping.started` callback on the first bake after the Editor has been started.

The problem is that when you start the Editor the backend is invalid until the Progressive Lightmapper clears itself (which takes a few Update ticks).
It looks to me that this check should look at `GetLightmapEditorSettings().GetBakeBackend()` (what the user has set) to do the validation and not at `GISceneManager::Get()->GetBakeBackend()` (which will be invalid for the first few ticks and generally is a bit weird, as the pipeline clearing itself could query the backend and is not really expecting to get invalid as a result).