Flutter Provider // setState() or markNeedsBuild() called during build

Question

I'm experimenting with Provider, but I get the error setState() or markNeedsBuild() called during build, if I change data and use notifyListeners().

What I do:

I initialize mainElementList holding a simple object when starting the App:

void main() {
  List mainElementList = [MainElement()];
  runApp(MyApp(
    mainElementList: mainElementList,
  ));
}

For now MainElement only has one property:

class MainElement {
  bool allTasksDone = false;
  }

Then, still in main.dart, I use mainElementlist as argument to set up a Provider (MainElementList) wrapping the MaterialApp:

return MultiProvider(
      providers: [
        ChangeNotifierProvider(
          create: (context) => MainElementList(widget.mainElementList),
        )
      ],
      child: MaterialApp(
        debugShowCheckedModeBanner: false,
        title: 'Three Things',
        initialRoute: '/',
        routes: {
          '/': (ctx) => TabsScreen(),
        },
      ),
    );

The Provider class:

class MainElementList with ChangeNotifier {
  List _mainElementList;

  MainElementList(this._mainElementList);

  get mainElementList {
    return [..._mainElementList];
  }
    
  void toggleAllTasksDone() {
    _mainElementList[0].allTasksDone = !_mainElementList[0].allTasksDone;
    notifyListeners();
  }
}

A couple of steps down the widget tree, I'm testing, whether I can toggle allTasksDone; I do this in a stateless (edit: typo; it's stateful) widget's build method before the return statement:

MainElementList mainElementList = Provider.of<MainElementList>(context);
print('Before: ${mainElementList.mainElementList[0].allTasksDone}');
mainElementList.toggleAllTasksDone();
print('After: ${mainElementList.mainElementList[0].allTasksDone}');

It works in so far that the value is toggled. But if I use notifyListeners in .toggleAllTasksDone, I get this:

> Exception caught by foundation library
> ════════════════════════════════ The following assertion was thrown
> while dispatching notifications for MainElementList: setState() or
> markNeedsBuild() called during build.
> 
> This _InheritedProviderScope<MainElementList?> widget cannot be marked
> as needing to build because the framework is already in the process of
> building widgets.  A widget can be marked as needing to be built
> during the build phase only if one of its ancestors is currently
> building. This exception is allowed because the framework builds
> parent widgets before children, which means a dirty descendant will
> always be built. Otherwise, the framework might not visit this widget
> during this build phase. The widget on which setState() or
> markNeedsBuild() was called was:
> _InheritedProviderScope<MainElementList?>
>     value: Instance of 'MainElementList'
>     listening to value The widget which was currently being built when the offending call was made was: MainElementWidget
>     dirty

Any help highly appreciated!

__ Edit: Full widget as per request:

class MainElementWidget extends StatefulWidget {
  const MainElementWidget({
    Key? key,
  }) : super(key: key);

  @override
  State<MainElementWidget> createState() => _MainElementWidgetState();
}

class _MainElementWidgetState extends State<MainElementWidget> {
  @override
  Widget build(BuildContext context) {
    //RESPONSIVENESS
    final availableWidth = context.read<DimensionsProvider>().availableWidth;
    final mainElementSize = availableWidth * 0.8;

    //COLORS
    final customColorScheme = context.read<CustomColors>().customColorScheme;

    //MAIN ELEMENT LIST
    MainElementList mainElementList = Provider.of<MainElementList>(context);
    print('Before: ${mainElementList.mainElementList[0].allTasksDone}');
    mainElementList.toggleAllTasksDone();
    print('After: ${mainElementList.mainElementList[0].allTasksDone}');

    return ClipRRect(
      borderRadius: BorderRadius.circular(15),
      child: Container(
        color: customColorScheme['Primary 3'],
        child: SizedBox(
          width: mainElementSize,
          height: mainElementSize,
        ),
      ),
    );
  }
}

Yes, I'm overusing maps ...sue me :)