Using Flutter Downloader plugin, after download app closes

Question

**I use Flutter Downloader Package After complete download some file , my app closes automatically and disconnecte to the android studio. Any one help me to find soltutions.

    final status = await Permission.storage.request();
            if (status.isGranted) {
              await downloadPDF();
            } 

downloadPDF() async {
    final externalDir = await getExternalStorageDirectory();
    taskId = await FlutterDownloader.enqueue(
      url: pdfURL,
      savedDir: externalDir.path,
      fileName: "Flamingo Order Details",
      showNotification: true,
      openFileFromNotification: true,
    );
  }

Here is my console error:

I/flutter (15913): Fatal: could not find callback
F/libc    (15913): Fatal signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x0 in tid 15956 (1.raster), pid 15913 (le.order_system)
*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** ***
Build fingerprint: 'motorola/chef/chef_sprout:10/QPTS30.61-18-16-8/03acd:user/release-keys'
Revision: 'pvt'
ABI: 'arm64'
Timestamp: 2021-04-23 16:39:38+0530
pid: 15913, tid: 15956, name: 1.raster  >>> com.example.order_system <<<
uid: 10870
signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x0
Cause: null pointer dereference
    x0  000000741e5a6478  x1  00000074841cfa00  x2  0000000000000001  x3  0000000000000000
    x4  0000000000000000  x5  00000000ffffffff  x6  00000000ffffffff  x7  0000000b96c6a75c
    x8  0000000080000081  x9  658adf78f7e836ee  x10 0000000000000000  x11 0000000000000000
    x12 0000000000000001  x13 000000747b19fe80  x14 0000007489a0a280  x15 0000000000000000
    x16 000000747b19b050  x17 0000007515c9787c  x18 000000741d05e000  x19 000000741e5a6478
    x20 00000074841cfa00  x21 0000000000000001  x22 0000000000000000  x23 00000074843db380
    x24 000000741e5a7020  x25 000000741e5a7020  x26 0000000000000000  x27 0000000000000001
    x28 0000000000000043  x29 000000741e5a6450
    sp  000000741e5a6420  lr  000000747b013f84  pc  000000747b01c378
backtrace:
      #00 pc 00000000001d8378  /vendor/lib64/egl/libGLESv2_adreno.so (BuildId: 22cc95e0051ae85072c405eeeeeb312d)
      #01 pc 00000000001cff80  /vendor/lib64/egl/libGLESv2_adreno.so (BuildId: 22cc95e0051ae85072c405eeeeeb312d)
      #02 pc 00000000000207b0  /system/lib64/libEGL.so (android::eglSwapBuffersWithDamageKHRImpl(void*, void*, int*, int)+316) (BuildId: 248ba7f2d80e7bb9952a20e1c3493c86)
      #03 pc 000000000001d0a8  /system/lib64/libEGL.so (eglSwapBuffers+80) (BuildId: 248ba7f2d80e7bb9952a20e1c3493c86)
      #04 pc 00000000012ec528  /data/app/com.example.order_system-8XYsushVsEZPOltQ3k8npA==/lib/arm64/libflutter.so (BuildId: e14966237eb013b063fed6484195268f7398b594)
Lost connection to device.

Solution

Maybe it late but it may help others. Recently I faced this error and I solved it. Your UI is rendering in Main isolate and your download events come from background isolate. Because codes in callback are run in the background isolate, so you have to handle the communication between two isolates. Usually, communication needs to take place to show download progress in the main UI. Implement the below code to handle communication:

import 'dart:isolate';
import 'dart:ui'; // You need to import these 2 libraries besides another libraries to work with this code

ReceivePort _port = ReceivePort();

@override
  void initState() {
    super.initState();

    IsolateNameServer.registerPortWithName(_port.sendPort, 'downloader_send_port');
    _port.listen((dynamic data) {
      String id = data[0];
      DownloadTaskStatus status = data[1];
      int progress = data[2];
      setState((){ });
    });

    FlutterDownloader.registerCallback(downloadCallback);
  }

@override
  void dispose() {
    IsolateNameServer.removePortNameMapping('downloader_send_port');
    super.dispose();
  }

  static void downloadCallback(String id, DownloadTaskStatus status, int progress) {
    final SendPort send = IsolateNameServer.lookupPortByName('downloader_send_port')!;
    send.send([id, status, progress]);
  }


void _download(String url) async {
    final status = await Permission.storage.request();

    if(status.isGranted) {
      final externalDir = await getExternalStorageDirectory();

      final id = await FlutterDownloader.enqueue(
          url: url,
          savedDir: externalDir!.path,
        showNotification: true,
        openFileFromNotification: true,
      );
    } else {
      print('Permission Denied');
    }
  }

Call _download(url) function in your button and you are ready to go.

N.B: I am using sound null safety in my app and for this reason I am using null aware operator in this line of code:

final SendPort send = IsolateNameServer.lookupPortByName('downloader_send_port')!;