[linter] prefer_non_nullable_members

[stephane-archer]

 class LutPreviewImagesChangeNotifier extends ChangeNotifier {
  final PreviewManager? _previewManager;

  LutPreviewImagesChangeNotifier(PreviewGenerator previewGenerator)
      : _previewManager = PreviewManager(previewGenerator);
}

this lint should suggest _previewManager to be of type final PreviewManager because it can't be null here