fix: trying to access addressable settings while editor is compiling

[Ale1]

Its documented limitation of AddressableAssetSettingsDefaultObject.Settings will return null if being accessed while Editor is compiling or updating. This provides a workaround to remove the null Settings error.

[starikcetin]

This works for AddressablesChangeListener. However, the generator also has the same problem, but the solution there won't be as simple. I want to figure out a solution there and merge the two together with this PR.

[Ale1]

I've updated this patch and used the same work-around for the SceneAssetProcessor (which is the other culprit of calling SceneDataMapsGenerator.Run() too soon).

Though in the long-run, maybe it makes more sense for the ScenDataMapsGenerator to delay itself instead of relying on its gentlemen callers to be polite with the timing. :-)

[starikcetin]

Yeah, what worries me is batchmode builds, because if quit flag is given, delays may misbehave. We need some rigorous testing there.

[starikcetin]

May be superseded by #87.

[starikcetin]

Superseded by #87.