saveToTempDir method
Implementation
Future<String> saveToTempDir() async {
Uint8List bytes = await this;
Directory tempDir = Directory.systemTemp;
String tempPath = tempDir.path;
final timestamp = DateTime.now().millisecondsSinceEpoch.toString();
String path = '$tempPath/sign_$timestamp.png';
await File(path).writeAsBytes(bytes);
return path;
}