getCachedFilenames method Null safety

Implementation

@override
List<String> getCachedFilenames() {
  final List<String> result = <String>[];
  final String? filename = getFilename();
  if (filename == null) {
    return result;
  }
  final String cacheFilename = getCacheFilename(filename);
  final String cacheTintableFilename = getCacheTintableFilename(filename);
  if (color == null) {
    result.add(cacheFilename);
    result.add(cacheTintableFilename);
  } else {
    result.add(cacheTintableFilename);
    result.add(cacheFilename);
  }
  return result;
}