"Unexpected null value." when running against Channel beta, 1.23.0-18.1.pre

[alexagat]

After upgrading the Flutter beta channel to latest:
[✓] Flutter (Channel beta, 1.23.0-18.1.pre, on Mac OS X 10.15.7 19H2 x86_64, locale en-US)

I get the following output in the console and an error in place of the PreloadPageView widget:

════════ Exception caught by widgets library ═══════════════════════════════════
The following TypeErrorImpl was thrown building NotificationListener<ScrollNotification>:
Unexpected null value.

The relevant error-causing widget was
NotificationListener<ScrollNotification>
../…/lib/preload_page_view.dart:572
When the exception was thrown, this was the stack
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/errors.dart 216:49      throw_
dart-sdk/lib/_internal/js_dev_runtime/private/ddc_runtime/operations.dart 518:63  nullCheck
packages/flutter/src/widgets/scroll_position.dart 166:31                          get pixels
packages/preload_page_view/preload_page_view.dart 269:16                          restoreScrollOffset
packages/flutter/src/widgets/scroll_position.dart 106:7                           new
...
════════════════════════════════════════════════════════════════════════════════

Any help or guidance is appreciated. I created a new project with only the example code to ensure it was not a conflict with my app-specific implementation.

[savs90]

This is related to the new dart version that flutter is using. They introduced null safety and as I can see updated the framework to the last syntax. I played a bit until make a fix.
Just exchange _PagePosition class with the code under:

class _PagePosition extends ScrollPositionWithSingleContext implements PageMetrics {
  _PagePosition({
    ScrollPhysics physics,
    ScrollContext context,
    this.initialPage = 0,
    bool keepPage = true,
    double viewportFraction = 1.0,
    ScrollPosition oldPosition,
  })  : assert(initialPage != null),
        assert(keepPage != null),
        assert(viewportFraction != null),
        assert(viewportFraction > 0.0),
        _viewportFraction = viewportFraction,
        _pageToUseOnStartup = initialPage.toDouble(),
        super(
          physics: physics,
          context: context,
          initialPixels: null,
          keepScrollOffset: keepPage,
          oldPosition: oldPosition,
        );

  final int initialPage;
  double _pageToUseOnStartup;

  @override
  double get viewportFraction => _viewportFraction;
  double _viewportFraction;

  set viewportFraction(double value) {
    if (_viewportFraction == value) return;
    final double oldPage = page;
    _viewportFraction = value;
    if (oldPage != null) forcePixels(getPixelsFromPage(oldPage));
  }

  double getPageFromPixels(double pixels, double viewportDimension) {
    return math.max(0.0, pixels) / math.max(1.0, viewportDimension * viewportFraction);
  }

  double getPixelsFromPage(double page) {
    return page * ((hasViewportDimension == true) ? viewportDimension : 0) * viewportFraction;
  }

  @override
  double get page => (hasPixels == false)
      ? null
      : getPageFromPixels(
          pixels.clamp((hasContentDimensions == true) ? minScrollExtent : null, (hasContentDimensions == true) ? maxScrollExtent : null),
          (hasViewportDimension == true) ? viewportDimension : null,
        );

  @override
  void saveScrollOffset() {
    PageStorage.of(context.storageContext)
        ?.writeState(context.storageContext, getPageFromPixels((hasPixels) ? pixels : null, (hasViewportDimension) ? viewportDimension : null));
  }

  @override
  void restoreScrollOffset() {
    if (hasPixels == true) {
      final double value = PageStorage.of(context.storageContext)?.readState(context.storageContext);
      if (value != null) _pageToUseOnStartup = value;
    }
  }

  @override
  bool applyViewportDimension(double viewportDimension) {
    final double oldViewportDimensions = (hasViewportDimension) ? this.viewportDimension : null;
    final bool result = super.applyViewportDimension(viewportDimension);
    final double oldPixels = (hasPixels) ? pixels : null;
    final double page = (oldPixels == null || oldViewportDimensions == 0.0) ? _pageToUseOnStartup : getPageFromPixels(oldPixels, oldViewportDimensions);
    final double newPixels = getPixelsFromPage(page);
    if (newPixels != oldPixels) {
      correctPixels(newPixels);
      return false;
    }
    return result;
  }

  @override
  PageMetrics copyWith({
    double minScrollExtent,
    double maxScrollExtent,
    double pixels,
    double viewportDimension,
    AxisDirection axisDirection,
    double viewportFraction,
  }) {
    return PageMetrics(
      minScrollExtent: minScrollExtent ?? ((hasContentDimensions) ? this.minScrollExtent : null),
      maxScrollExtent: maxScrollExtent ?? ((hasContentDimensions) ? this.maxScrollExtent : null),
      pixels: pixels ?? ((hasPixels) ? this.pixels : null),
      viewportDimension: viewportDimension ?? ((hasViewportDimension) ? this.viewportDimension : null),
      axisDirection: axisDirection ?? this.axisDirection,
      viewportFraction: viewportFraction ?? this.viewportFraction,
    );
  }
}

This should take in consideration the new syntax. Please test and give a review so other people with the same problem could fix it.

[alexagat]

@savs90 this is working for Channel beta, 1.23.0-18.1.pre. Please consider adding this change in.

[crtl]

@savs90 Thanks your saving my day. Working on master.
I have created a fork and applied your chages their it is avalable with:

preload_page_view:
    git:
      url: "https://github.com/crtl/preload_page_view.git"
      ref: "fix-null-check-operators"

[Bonsai11]

And once again we see the downsides of plugins.
There shouldn't be a fork that fixes this. It should be fixed here and updated on pub.dev.
@octomato @wrbl606

In a perfect world, this would be added in the actual flutter widget, so nobody needs a plugin for this.

[octomato]

@Bonsai11 Thanks for the reminder, will review the code and create a PR later on

[octomato]

@Bonsai11 @savs90 @crtl Please review the PR #21

[octomato]

Fixed it.

[Bonsai11]

Thanks, thats very good, do you have any plans yet to update this on pub.dev?