Adds a lock to the fetch function.

locked_fetch.dart

var _fetching = false;

fetchAndUpdateImage([String url = 'https://picsum.photos/300/200']) async {
  if(_fetching) {
    return;
  }
  _fetching = true;
  final response = await http.get(url);
  floop['image'] = TransitionImage(Image.memory(response.bodyBytes));
  _fetching = false;
}