Running page render on Isolate throw 'Null check operator used on a null value'

[ncvescera]

Describe the bug I am trying to render all the pdf's pages in the background via Dart Isolate but when I call the render function (page.render()) I have this error: Null check operator used on a null value. Is that a bug or am I doing something wrong ??

Platform: Android Flutter Version: 2.10.5

Full error message:

E/flutter ( 9510): [ERROR:flutter/runtime/dart_isolate.cc(1111)] Unhandled exception:
E/flutter ( 9510): Null check operator used on a null value
E/flutter ( 9510): #0      BasicMessageChannel.binaryMessenger                   package:flutter/…/services/platform_channel.dart:52
E/flutter ( 9510): #1      BasicMessageChannel.send                              package:flutter/…/services/platform_channel.dart:60
E/flutter ( 9510): #2      PdfxApi.renderPage                                    package:pdfx/…/io/pigeon.dart:634
E/flutter ( 9510): #3      PdfPageImagePigeon.render                             package:pdfx/…/io/platform_pigeon.dart:253
E/flutter ( 9510): #4      PdfPagePigeon.render.<anonymous closure>              package:pdfx/…/io/platform_pigeon.dart:158
E/flutter ( 9510): #5      PdfPagePigeon.render.<anonymous closure>              package:pdfx/…/io/platform_pigeon.dart:151
E/flutter ( 9510): #6      BasicLock.synchronized                                package:synchronized/src/basic_lock.dart:31
E/flutter ( 9510): #7      PdfPagePigeon.render                                  package:pdfx/…/io/platform_pigeon.dart:151
E/flutter ( 9510): #8      renderer                                              package:keep_reading/API/pdf_manager.dart:116
E/flutter ( 9510): <asynchronous suspension>

My code:

void preloadPdfPages(String filePath, Function callback) async {
  final ReceivePort receivePort = ReceivePort();
  await Isolate.spawn(renderer, receivePort.sendPort);

  var sendPort = await receivePort.first;

  final document = await PdfDocument.openFile(filePath); // Pages start at 1
  final totalPages = document.pagesCount;

  List<Image> pages = <Image>[];

  for (int i = 1; i <= totalPages; i++) {
    PdfPage page = await document.getPage(i);

    PdfPageImage? pageImage =
        await sendReceive(sendPort, page) as PdfPageImage?;

    if (pageImage != null) {
      pages.add(
        Image(
          image: MemoryImage(pageImage.bytes),
        ),
      );
    }

    await page.close();
  }

  await sendReceive(sendPort, null); // Close the isolate
  await document.close();

  callback(pages);
}

// the entry point for the isolate
void renderer(SendPort sendPort) async {
  // Open the ReceivePort for incoming messages.
  ReceivePort port = ReceivePort();

  // Notify any other isolates what port this isolate listens to.
  sendPort.send(port.sendPort);

  await for (var msg in port) {
    var data = msg[0];
    SendPort replyTo = msg[1];

    if (data == null) {
      port.close();
    }

    PdfPage page = data as PdfPage;
    debugPrint('w: ${page.width} h: ${page.height}');

    final double width = page.width * 2;
    final double height = page.height * 2;

    PdfPageImage? pageImage = await page.render(   // error here
      width: width,
      height: height,
      format: PdfPageImageFormat.png,
      backgroundColor: '#ffffff',
    );

    replyTo.send(pageImage);
  }
}

/// sends a message on a port, receives the response,
/// and returns the message
Future sendReceive(SendPort port, msg) {
  ReceivePort response = ReceivePort();
  port.send([msg, response.sendPort]);
  return response.first;
}

[SergeShkurko]

Hi Platform Android / iOS? Flutter version?

[ncvescera]

Sorry, I forgot to add this on the issue !

I am on Android with Flutter 2.10.5