Save and load image to /data/user in flutter

I would like to let the user choose several images. After confirmation I want to concatenate them and then share it with other apps. Concatenating works fine, I have written the final Image as a ByteStream using the image/image.dart library (image: ^2.1.4). For sharing I am using the esys library (esys_flutter_share: ^1.0.2).

import 'package:image/image.dart' as pkg_img;

Future<String> _joinImages(List<Type> ikonList) async {
  Future<List> imageList() async {
    // some logic to get the data
  }
  pkg_img.Image mergedImage;
  final documentDirectory = await getApplicationDocumentsDirectory();
  final String filename = "test.png";
  final String fullFilePath = join(documentDirectory.path, "${filename}");
  FutureBuilder(
    future: imageList(),
    builder: (context, snapshot) {
      if (!snapshot.hasData) {
        return CircularProgressIndicator();
      } else {
        List<pkg_img.Image> myList = snapshot.data;
        mergedImage = someLogicToConcatenateImages(myList);

        final file = new File(fullFilePath);
        file.writeAsBytesSync(pkg_img.encodePng(mergedImage));
        return null;
      }
  });

  print(fullFilePath);
  return await join(fullFilePath);

}

_shareImage(String fileName) async {
  Uint8List bytes = await _readFileByte(fileName);
  await Share.file('esys image', 'esys.png', bytes, 'image/png',
      text: 'Sent from My App.');
}

Future<Uint8List> _readFileByte(String filePath) async {
  Uri myUri = Uri.parse(filePath);
  File myFile = new File.fromUri(myUri);
  Uint8List bytes;
  await myFile.readAsBytes().then((value) {
    bytes = Uint8List.fromList(value);
    print('reading of bytes is completed');
  });
  return bytes;
}

This whole code is being called when clicking on a button like so:

RaisedButton(
  onPressed: () {
    _joinImages(_ikonList).then(_shareImage);
  },
  child: Text('Export!'),
),

The error I run into is the following:

I/flutter (13768): File: '/data/user/0/com.myapp/app_flutter/test.png'
E/flutter (13768): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: FileSystemException: Cannot open file, path = '/data/user/0/com.myapp/app_flutter/test.png' (OS Error: No such file or directory, errno = 2)
E/flutter (13768): #0      _File.open.<anonymous closure> (dart:io/file_impl.dart:366:9)
E/flutter (13768): #1      _rootRunUnary (dart:async/zone.dart:1134:38)
E/flutter (13768): #2      _CustomZone.runUnary (dart:async/zone.dart:1031:19)
E/flutter (13768): #3      _FutureListener.handleValue (dart:async/future_impl.dart:139:18)
E/flutter (13768): #4      Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:680:45)
E/flutter (13768): #5      Future._propagateToListeners (dart:async/future_impl.dart:709:32)
E/flutter (13768): #6      Future._completeWithValue (dart:async/future_impl.dart:524:5)
E/flutter (13768): #7      Future._asyncComplete.<anonymous closure> (dart:async/future_impl.dart:554:7)
E/flutter (13768): #8      _rootRun (dart:async/zone.dart:1126:13)
E/flutter (13768): #9      _CustomZone.run (dart:async/zone.dart:1023:19)
E/flutter (13768): #10     _CustomZone.runGuarded (dart:async/zone.dart:925:7)
E/flutter (13768): #11     _CustomZone.bindCallbackGuarded.<anonymous closure> (dart:async/zone.dart:965:23)
E/flutter (13768): #12     _microtaskLoop (dart:async/schedule_microtask.dart:43:21)
E/flutter (13768): #13     _startMicrotaskLoop (dart:async/schedule_microtask.dart:52:5)
E/flutter (13768):