Flutter 2.0: The return Type <List<LatLng>> isn’t a 'void', as required by the closure's context

I am getting the error The return Type <List<LatLng>> isn’t a 'void', as required by the closure's context regarding the line return polygonList; in this button widget:

Widget _markerButton(BuildContext context) {
  return IconButton(
    icon: Icon(Icons.favorite, color: _list ? Colors.green : Colors.red),
    onPressed: () {
      setState(() {
        _list = !_list;
        numberOfMarkers(latlng)[0];
        realtyTap = true;
      });
      return polygonList;
    },
  );
}

It worked excellent before. This error appeared when I switched to Flutter 2.0 and sdk: "> = 2.12.0 <3.0.0” .

I looked at the answers to questions from people with similar difficulties, but it did not help me.

The whole page code is:

import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';

import 'package:flutter_map/flutter_map.dart';
import 'package:latlong/latlong.dart';


class HomePage extends StatefulWidget {
  final Function state;
  HomePage(this.state);

  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  late MapController _mapController;
  late LatLng latlng;

  List<Marker> markers = [];
  List<LatLng> polygonList = [];

  bool singleTap = false;
  bool _list = false;

  double maxZoom = 6.0;
  double minZoom = 4.0;

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

  Widget _markerButton(BuildContext context) {
    return IconButton(
      icon: Icon(Icons.favorite, color: _list ? Colors.green : Colors.red),
      onPressed: () {
        setState(() {
          _list = !_list;
        });
        return polygonList;
      },
    );
  }

  @override
  Widget build(BuildContext context) {

    return Scaffold(
      body: FlutterMap(
        mapController: _mapController,
        options: MapOptions(
            center: LatLng(40.000000, 70.000000),
            zoom: 4,
            onTap: (latlng) {
              if (singleTap) {
                setState(() {
                  if (markers.length == 0) {
                    markers.add(Marker(
                        point: latlng,
                        builder: (context) => _markerButton(context)));
                  } else {
                    markers.add(
                      Marker(
                        point: latlng,
                        builder: (ctx) => const Icon(
                          Icons.fiber_manual_record_rounded,
                          color: Colors.red,
                          size: 15.0,
                        ),
                      ),
                    );
                  }
                  polygonList.add(latlng);
                });
              }
            }),
        layers: [
          TileLayerOptions(
              urlTemplate: "https://{s}.tile.openstreetmap.org/{z}/{x}/{y}.png",
              subdomains: ['a', 'b', 'c']),
          PolygonLayerOptions(
            polygons: [
              Polygon(
                points: polygonList,
                borderStrokeWidth: 3.0,
              ),
            ],
          ),
        ],
      ),
    );
  }
}

What should be happened here: if the user clicks for the second time the point from which s/he started building the polygon on the map, the button change its color, the building stops, and a list of points in the corners of the constructed polygon is returned.

I would be grateful if you could give me some advice on how to fix this issue.

Dart 2.12 added stricter type checking to catch cases where callbacks return unused values.

IconButton.onPressed is a VoidCallback?; that is, if it isn't null, it is a function that returns nothing.

You are supplying an anonymous function that has return polygonList; in it. That returned value can never be used, so the return statement is useless. Simply remove it.