build method Null safety

Implementation

@override
Widget build(BuildContext context) {
  final List<String> cachedFilenames = getCachedFilenames();
  if (cachedFilenames.isEmpty) {
    return getDefaultUnknown();
  }
  return SvgPicture.network(
    iconUrl!,
    color: color,
    width: width,
    height: height,
    fit: BoxFit.contain,
    placeholderBuilder: (BuildContext context) => displayAssetWhileWaiting
        ? SvgAsyncAsset(
            AssetCacheHelper(
              cachedFilenames,
              iconUrl!,
              width: width,
              height: height,
              color: color,
            ),
          )
        : getCircularProgressIndicator(),
  );
}