What dose BINARY_EVENT mean? I frequently get error for this reason

[Aerdax]

i notice that the function onpacket(packet){} is used to classify server messages,and have 5 case in there,i think only EVENT is custom message,but i often receive BINARY_EVENT with null data,and get an error,i dont know why.my server used SpringBoot+netty-socket-io,and The following is the error message,Thanks for answering me!

E/flutter (19571): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: NoSuchMethodError: Closure call with mismatched arguments: function 'call' E/flutter (19571): Receiver: Closure: (String, [dynamic]) => void from Function 'emit':. E/flutter (19571): Tried calling: call() E/flutter (19571): Found: call(String, [dynamic]) => void E/flutter (19571): #0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5) E/flutter (19571): #1 Function._apply (dart:core-patch/function_patch.dart:12:68) E/flutter (19571): #2 Function.apply (dart:core-patch/function_patch.dart:33:12) E/flutter (19571): #3 Socket.onevent (package:socket_io_client/src/socket.dart:278:18) E/flutter (19571): #4 Socket.onpacket (package:socket_io_client/src/socket.dart:238:9) E/flutter (19571): #5 EventEmitter.emit. (package:socket_io_common/src/util/event_emitter.dart:52:14) E/flutter (19571): #6 List.forEach (dart:core-patch/growable_array.dart:282:8) E/flutter (19571): #7 EventEmitter.emit (package:socket_io_common/src/util/event_emitter.dart:51:11) E/flutter (19571): #8 Manager.ondecoded (package:socket_io_client/src/manager.dart:320:5) E/flutter (19571): #9 EventEmitter.emit. (package:socket_io_common/src/util/event_emitter.dart:52:14) E/flutter (19571): #10 List.forEach (dart:core-patch/growable_array.dart:282:8) E/flutter (19571): #11 EventEmitter.emit (package:socket_io_common/src/util/event_emitter.dart:51:11) E/flutter (19571): #12 Decoder.add (package:socket_io_common/src/parser/parser.dart:173:16) E/flutter (19571): #13 Manager.ondata (package:socket_io_client/src/manager.dart:311:13) E/flutter (19571): #14 EventEmitter.emit. (package:socket_io_common/src/util/event_emitter.dart:52:14) E/flutter (19571): #15 List.forEach (dart:core-patch/growable_array.dart:282:8) E/flutter (19571): #16 EventEmitter.emit (package:socket_io_common/src/util/event_emitter.dart:51:11) E/flutter (19571): #17 Socket.onPacket (package:socket_io_client/src/engine/socket.dart:467:11) E/flutter (19571): #18 Socket.setTransport. (package:socket_io_client/src/engine/socket.dart:292:34) E/flutter (19571): #19 EventEmitter.emit. (package:socket_io_common/src/util/event_emitter.dart:52:14) E/flutter (19571): #20 List.forEach (dart:core-patch/growable_array.dart:282:8) E/flutter (19571): #21 EventEmitter.emit (package:socket_io_common/src/util/event_emitter.dart:51:11) E/flutter (19571): #22 Transport.onPacket (package:socket_io_client/src/engine/transport/transport.dart:128:5) E/flutter (19571): #23 Transport.onData (package:socket_io_client/src/engine/transport/transport.dart:122:5) E/flutter (19571): #24 IOWebSocketTransport.addEventListeners. (package:socket_io_client/src/engine/transport/io_websocket_transport.dart:67:7) E/flutter (19571): #25 _rootRunUnary (dart:async/zone.dart:1192:38) E/flutter (19571): #26 _CustomZone.runUnary (dart:async/zone.dart:1085:19) E/flutter (19571): #27 _CustomZone.runUnaryGuarded (dart:async/zone.dart:987:7) E/flutter (19571): #28 _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:339:11) E/flutter (19571): #29 _BufferingStreamSubscription._add (dart:async/stream_impl.dart:266:7) E/flutter (19571): #30 _SyncStreamControllerDispatch._sendData (dart:async/stream_controller.dart:779:19) E/flutter (19571): #31 _StreamController._add (dart:async/stream_controller.dart:655:7) E/flutter (19571): #32 _StreamController.add (dart:async/stream_controller.dart:597:5) E/flutter (19571): #33 new _WebSocketImpl._fromSocket. (dart:_http/websocket_impl.dart:1146:21) E/flutter (19571): #34 _rootRunUnary (dart:async/zone.dart:1192:38) E/flutter (19571): #35 _CustomZone.runUnary (dart:async/zone.dart:1085:19) E/flutter (19571): #36 _CustomZone.runUnaryGuarded (dart:async/zone.dart:987:7) E/flutter (19571): #37 _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:339:11) E/flutter (19571): #38 _BufferingStreamSubscription._add (dart:async/stream_impl.dart:266:7) E/flutter (19571): #39 _SinkTransformerStreamSubscription._add (dart:async/stream_transformers.dart:70:11) E/flutter (19571): #40 _EventSinkWrapper.add (dart:async/stream_transformers.dart:17:11) E/flutter (19571): #41 _WebSocketProtocolTransformer._messageFrameEnd (dart:_http/websocket_impl.dart:341:22) E/flutter (19571): #42 _WebSocketProtocolTransformer.add (dart:_http/websocket_impl.dart:233:46) E/flutter (19571): #43 _SinkTransformerStreamSubscription._handleData (dart:async/stream_transformers.dart:122:24) E/flutter (19571): #44 _rootRunUnary (dart:async/zone.dart:1192:38) E/flutter (19571): #45 _CustomZone.runUnary (dart:async/zone.dart:1085:19) E/flutter (19571): #46 _CustomZone.runUnaryGuarded (dart:async/zone.dart:987:7) E/flutter (19571): #47 _BufferingSt