NoSuchMethodError: The getter 'cls' was called on null - flutter

Question

How can I resolve the problem as below ?
I run my application normally, it's run well.
But when I've set break point, then tried to perform debugging my code.
It always shows the error messages which do not come from my code, debugger is interrupted as well.
Unhandled exception:
NoSuchMethodError: The getter 'cls' was called on null.
Receiver: null
Tried calling: cls
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:50:5)
#1 BodyBuilder.lookupInstanceMember (package:front_end/src/fasta/kernel/body_builder.dart:1604:30)
#2 BodyBuilder.scopeLookup (package:front_end/src/fasta/kernel/body_builder.dart:1764:13)
#3 BodyBuilder.handleIdentifier (package:front_end/src/fasta/kernel/body_builder.dart:1700:12)
#4 Parser.ensureIdentifier (package:front_end/src/fasta/parser/parser.dart:2165:14)
#5 Parser.parseSend (package:front_end/src/fasta/parser/parser.dart:4932:32)
#6 Parser.parseSendOrFunctionLiteral (package:front_end/src/fasta/parser/parser.dart:4675:12)
#7 Parser.parsePrimary (package:front_end/src/fasta/parser/parser.dart:4233:14)
#8 Parser.parseUnaryExpression (package:front_end/src/fasta/parser/parser.dart:4172:12)
#9 Parser.parsePrecedenceExpression (package:front_end/src/fasta/parser/parser.dart:3961:13)
#10 Parser.parseExpression (package:front_end/src/fasta/parser/parser.dart:3933:13)
#11 BodyBuilder.parseSingleExpression (package:front_end/src/fasta/kernel/body_builder.dart:1111:20)
#12 SourceLoader.buildExpression (package:front_end/src/fasta/source/source_loader.dart:327:21)
#13 IncrementalCompiler.compileExpression. (package:front_end/src/fasta/incremental_compiler.dart:795:61)
#14 CompilerContext.runInContext..
(package:front_end/src/fasta/compiler_context.dart:122:46)
#15 new Future.sync (dart:async/future.dart:224:31)
#16 CompilerContext.runInContext. (package:front_end/src/fasta/compiler_context.dart:122:19)
#17 _rootRun (dart:async/zone.dart:1124:13)
#18 _CustomZone.run (dart:async/zone.dart:1021:19)
#19 _runZoned (dart:async/zone.dart:1516:10)
#20 runZoned (dart:async/zone.dart:1463:12)
#21 CompilerContext.runInContext (package:front_end/src/fasta/compiler_context.dart:121:12)
#22 IncrementalCompiler.compileExpression (package:front_end/src/fasta/incremental_compiler.dart:726:26)
#23 IncrementalCompiler.compileExpression (package:vm/incremental_compiler.dart:156:23)
#24 FrontendCompiler.compileExpression (package:vm/frontend_server.dart:527:44)
#25 _FlutterFrontendCompiler.compileExpression (package:frontend_server/server.dart:62:22)
#26 listenAndCompile. (package:vm/frontend_server.dart:812:20)
#27 _RootZone.runUnaryGuarded (dart:async/zone.dart:1314:10)
#28 _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:336:11)
#29 _BufferingStreamSubscription._add (dart:async/stream_impl.dart:263:7)
#30 _SinkTransformerStreamSubscription._add (dart:async/stream_transformers.dart:68:11)
#31 _EventSinkWrapper.add (dart:async/stream_transformers.dart:15:11)
#32 _StringAdapterSink.add (dart:convert/string_conversion.dart:236:11)
#33 _LineSplitterSink._addLines (dart:convert/line_splitter.dart:150:13)
#34 _LineSplitterSink.addSlice (dart:convert/line_splitter.dart:125:5)
#35 StringConversionSinkMixin.add (dart:convert/string_conversion.dart:163:5)
#36 _SinkTransformerStreamSubscription._handleData (dart:async/stream_transformers.dart:120:24)
#37 _RootZone.runUnaryGuarded (dart:async/zone.dart:1314:10)
#38 _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:336:11)
#39 _BufferingStreamSubscription._add (dart:async/stream_impl.dart:263:7)
#40 _SinkTransformerStreamSubscription._add (dart:async/stream_transformers.dart:68:11)
#41 _EventSinkWrapper.add (dart:async/stream_transformers.dart:15:11)
#42 _StringAdapterSink.add (dart:convert/string_conversion.dart:236:11)
#43 _StringAdapterSink.addSlice (dart:convert/string_conversion.dart:241:7)
#44 _Utf8ConversionSink.addSlice (dart:convert/string_conversion.dart:312:20)
#45 _Utf8ConversionSink.add (dart:convert/string_conversion.dart:305:5)
#46 _ConverterStreamEventSink.add (dart:convert/chunked_conversion.dart:72:18)
#47 _SinkTransformerStreamSubscription._handleData (dart:async/stream_transformers.dart:120:24)
#48 _RootZone.runUnaryGuarded (dart:async/zone.dart:1314:10)
#49 _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:336:11)
#50 _BufferingStreamSubscription._add (dart:async/stream_impl.dart:263:7)
#51 _SyncStreamController._sendData (dart:async/stream_controller.dart:764:19)
#52 _StreamController._add (dart:async/stream_controller.dart:640:7)
#53 _StreamController.add (dart:async/stream_controller.dart:586:5)
#54 _Socket._onData (dart:io-patch/socket_patch.dart:1786:41)
#55 _RootZone.runUnaryGuarded (dart:async/zone.dart:1314:10)
#56 _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:336:11)
#57 _BufferingStreamSubscription._add (dart:async/stream_impl.dart:263:7)
#58 _SyncStreamController._sendData (dart:async/stream_controller.dart:764:19)
#59 _StreamController._add (dart:async/stream_controller.dart:640:7)
#60 _StreamController.add (dart:async/stream_controller.dart:586:5)
#61 new _RawSocket. (dart:io-patch/socket_patch.dart:1335:33)
#62 _NativeSocket.issueReadEvent.issue (dart:io-patch/socket_patch.dart:856:14)
#63 _microtaskLoop (dart:async/schedule_microtask.dart:41:21)
#64 _startMicrotaskLoop (dart:async/schedule_microtask.dart:50:5)
#65 _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:116:13)
#66 _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:173:5)
My code:
main.dart
void main() async {
Flame.images.loadAll(<String>[
'level0.jpg'
]);
Util flameUtil = Util();
await flameUtil.fullScreen();
await flameUtil.setOrientation(DeviceOrientation.portraitUp);
MyGame game = MyGame ();
runApp(game.widget);
....
mygame.dart:
......
MyGame() {
sprite = Sprite('level0.jpg');// Break point
}
.............
note:
I am using Android studio 3.1
flutter veriosn 1.7.8
channel stable
Thanks a lot!!!