getLastImageUrl function Null safety

Implementation

Future<String> getLastImageUrl() async {
  String url = '';
  var response = await command('listFiles',
      parameters: {'fileType': 'image', 'entryCount': 1, 'maxThumbSize': 0});
  var mapResponse = jsonDecode(response);
  url = mapResponse['results']['entries'][0]['fileUrl'];
  return url.toString();
}