由于我一直在使用google polyline插件，我的应用程序抛出了此异常，我尝试更改版本，但仍然无法正常工作，有人可以帮我解决这个问题，这里是完整的异常：
[ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: NoSuchMethodError: The getter 'iterator' was called on null.
E/flutter (23542): Receiver: null
E/flutter (23542): Tried calling: iterator
E/flutter (23542): #0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
E/flutter (23542): #1 new List.from (dart:core-patch/array_patch.dart:54:19)
E/flutter (23542): #2 new List.of (dart:core/list.dart:142:65)
E/flutter (23542): #3 Polyline.clone (package:google_maps_flutter/src/polyline.dart:168:18)
E/flutter (23542): #4 _keyByPolylineId.<anonymous closure> (package:google_maps_flutter/src/polyline.dart:251:41)
E/flutter (23542): #5 MappedIterator.moveNext (dart:_internal/iterable.dart:395:18)
E/flutter (23542): #6 MapMixin.addEntries (dart:collection/maps.dart:172:23)
E/flutter (23542): #7 new Map.fromEntries (dart:core/map.dart:187:17)
E/flutter (23542): #8 _keyByPolylineId (package:google_maps_flutter/src/polyline.dart:249:3)
E/flutter (23542): #9 new _PolylineUpdates.from (package:google_maps_flutter/src/polyline_updates.dart:24:9)
E/flutter (23542): #10 _GoogleMapState._updatePolylines (package:google_maps_flutter/src/google_map.dart:297:26)
E/flutter (23542): <asynchronous suspension>
E/flutter (23542): #11 _GoogleMapState.didUpdateWidget (package:google_maps_flutter/src/google_map.dart:260:5)
E/flutter (23542): #12 StatefulElement.update (package:flutter/src/widgets/framework.dart:4396:58)
E/flutter (23542): #13 Element.updateChild (package:flutter/src/widgets/framework.dart:2977:15)
E/flutter (23542): #14 SingleChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5452:14)
E/flutter (23542): #15 Element.updateChild (package:flutter/src/widgets/framework.dart:2977:15)
E/flutter (23542): #16 RenderObjectElement.updateChildren (package:flutter/src/widgets/framework.dart:5161:32)
E/flutter (23542): #17 MultiChildRenderObjectElement.update (package:flutter/src/widgets/framework.dart:5561:17)
E/flutter (23542): #18 Element.updateChild (package:flutter/src/widgets/framework.dart:2977:15)
E/flutter (23542): #19 _LayoutBuilderElement._layout.<anonymous closure> (package:flutter/src/widgets/layout_builder.dart:115:18)
E/flutter (23542): #20 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2412:19)
E/flutter (23542): #21 _LayoutBuilderElement._layout (package:flutter/src/widgets/layout_builder.dart:95:11)
E/flutter (23542): #22 RenderObject.invokeLayoutCallback.<anonymous closure> (package:flutter/src/rendering/object.dart:1823:58)
E/flutter (23542): #23 PipelineOwner._enableMutationsToDirtySubtrees (package:flutter/src/rendering/object.dart:875:15)
E/flutter (23542): #24 RenderObject.invokeLayoutCallback (package:flutter/src/rendering/object.dart:1823:13)
E/flutter (23542): #25 RenderConstrainedLayoutBuilder.layoutAndBuildChild (package:flutter/src/widgets/layout_builder.dart:173:5)
E/flutter (23542): #26 _RenderLayoutBuilder.performLayout (package:flutter/src/widgets/layout_builder.dart:240:5)
E/flutter (23542): #27 RenderObject.layout (package:flutter/src/rendering/object.dart:1724:7)
E/flutter (23542): #28 MultiChildLayoutDelegate.layoutChild (package:flutter/src/rendering/custom_layout.dart:163:11)
E/flutter (23542): #29 _ScaffoldLayout.performLayout (package:flutter/src/material/scaffold.dart:477:7)
E/flutter (23542): #30 MultiChildLayoutDelegate._callPerformLayout (package:flutter/src/rendering/custom_layout.dart:232:7)
E/flutter (23542): #31 RenderCustomMultiChildLayoutBox.performLayout (package:flutter/src/rendering/custom_layout.dart:391:14)
E/flutter (23542): #32 RenderObject._layoutWithoutResize (package:flutter/src/rendering/object.dart:1584:7)
E/flutter (23542): #33 PipelineOwner.flushLayout (package:flutter/src/rendering/object.dart:844:18)
E/flutter (23542): #34 RendererBinding.drawFrame (package:flutter/src/rendering/binding.dart:344:19)
E/flutter (23542): #35 WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:774:13)
E/flutter (23542): #36 RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:283:5)
E/flutter (23542): #37 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1102:15)
E/flutter (23542): #38 SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:1041:9)
E/flutter (23542): #39 SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:957:5)
E/flutter (23542): #40 _rootRun (dart:async/zone.dart:1126:13)
E/flutter (23542): #41 _CustomZone.run (dart:async/zone.dart:1023:19)
E/flutter (23542): #42 _CustomZone.runGuarded (dart:async/zone.dart:925:7)
E/flutter (23542): #43 _invoke (dart:ui/hooks.dart:259:10)
E/flutter (23542): #44 _drawFrame (dart:ui/hooks.dart:217:3)
这是我的代码，我从官方插件网站复制了此示例，如果可以测试它的话，将非常有用：
import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';
import 'package:google_maps_flutter/google_maps_flutter.dart';
import 'package:google_map_polyline/google_map_polyline.dart';
void main() => runApp(MyApp());
class MyApp extends StatefulWidget {
@override
_MyAppState createState() => _MyAppState();
}
class _MyAppState extends State<MyApp> {
int _polylineCount = 1;
Map<PolylineId, Polyline> _polylines = <PolylineId, Polyline>{};
GoogleMapController _controller;
GoogleMapPolyline _googleMapPolyline =
new GoogleMapPolyline(apiKey: "YOUR KEY HERE");
List<List<PatternItem>> patterns = <List<PatternItem>>[
<PatternItem>[], //line
<PatternItem>[PatternItem.dash(30.0), PatternItem.gap(20.0)], //dash
<PatternItem>[PatternItem.dot, PatternItem.gap(10.0)], //dot
<PatternItem>[
//dash-dot
PatternItem.dash(30.0),
PatternItem.gap(20.0),
PatternItem.dot,
PatternItem.gap(20.0)
],
];
LatLng _mapInitLocation = LatLng(40.683337, -73.940432);
LatLng _originLocation = LatLng(40.677939, -73.941755);
LatLng _destinationLocation = LatLng(40.698432, -73.924038);
bool _loading = false;
_onMapCreated(GoogleMapController controller) {
setState(() {
_controller = controller;
});
}
_getPolylinesWithLocation() async {
_setLoadingMenu(true);
List<LatLng> _coordinates =
await _googleMapPolyline.getCoordinatesWithLocation(
origin: _originLocation,
destination: _destinationLocation,
mode: RouteMode.driving);
setState(() {
_polylines.clear();
});
_addPolyline(_coordinates);
_setLoadingMenu(false);
}
_getPolylinesWithAddress() async {
_setLoadingMenu(true);
List<LatLng> _coordinates =
await _googleMapPolyline.getPolylineCoordinatesWithAddress(
origin: '55 Kingston Ave, Brooklyn, NY 11213, USA',
destination: '8007 Cypress Ave, Glendale, NY 11385, USA',
mode: RouteMode.driving);
setState(() {
_polylines.clear();
});
_addPolyline(_coordinates);
_setLoadingMenu(false);
}
_addPolyline(List<LatLng> _coordinates) {
PolylineId id = PolylineId("poly$_polylineCount");
Polyline polyline = Polyline(
polylineId: id,
patterns: patterns[0],
color: Colors.blueAccent,
points: _coordinates,
width: 10,
onTap: () {});
setState(() {
_polylines[id] = polyline;
_polylineCount++;
});
}
_setLoadingMenu(bool _status) {
setState(() {
_loading = _status;
});
}
@override
Widget build(BuildContext context) {
return MaterialApp(
darkTheme: ThemeData(brightness: Brightness.dark),
home: Scaffold(
appBar: AppBar(
title: Text('Map Polyline'),
),
body: Container(
child: LayoutBuilder(
builder: (context, cont) {
return Column(
children: <Widget>[
SizedBox(
width: MediaQuery.of(context).size.width,
height: MediaQuery.of(context).size.height - 175,
child: GoogleMap(
onMapCreated: _onMapCreated,
polylines: Set<Polyline>.of(_polylines.values),
initialCameraPosition: CameraPosition(
target: _mapInitLocation,
zoom: 15,
),
),
),
Expanded(
child: Align(
alignment: Alignment.center,
child: Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: <Widget>[
RaisedButton(
child: Text('Polylines wtih Location'),
onPressed: _getPolylinesWithLocation,
),
RaisedButton(
child: Text('Polylines wtih Address'),
onPressed: _getPolylinesWithAddress,
),
],
)),
),
],
);
},
),
),
floatingActionButtonLocation: FloatingActionButtonLocation.centerDocked,
floatingActionButton: _loading
? Container(
color: Colors.black.withOpacity(0.75),
child: Center(
child: Text(
'Loading...',
style: TextStyle(fontSize: 20, fontWeight: FontWeight.bold),
),
),
)
: Container(),
),
debugShowCheckedModeBanner: false,
);
}
}
origin: '55 Kingston Ave, Brooklyn, NY 11213, USA',
destination: '8007 Cypress Ave, Glendale, NY 11385, USA',
至：
origin: '55 Kingston Ave Brooklyn NY 11213', destination: '8007 Cypress Ave Glendale NY 11385',