oauth.dart's Error message

errorOfOauth.dart.txt

Uncaught Error: The null object does not have a method 'expand'.

NoSuchMethodError: method not found: 'expand'
Receiver: null
Arguments: [Closure: (int) => dynamic]
Stack Trace: 
#0      Object.noSuchMethod (dart:core-patch/object_patch.dart:45)
#1      oauthEncode (package:oauth/src/core.dart:29:39)
#2      Parameter.Parameter (package:oauth/src/core.dart:73:29)
#3      mapParameters.<anonymous closure> (package:oauth/src/core.dart:108:34)
#4      MappedIterator.moveNext (dart:_internal/iterable.dart:374)
#5      List.addAll (dart:core-patch/growable_array.dart:174)
#6      generateParameters (package:oauth/client.dart:35:23)
#7      signRequest (package:oauth/client.dart:71:34)
#8      Client.send.<anonymous closure> (package:oauth/client.dart:108:18)
#9      _RootZone.runUnary (dart:async/zone.dart:1088)
#10     _Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:488)
#11     _Future._propagateToListeners (dart:async/future_impl.dart:571)
#12     _Future._completeWithValue (dart:async/future_impl.dart:331)
#13     _Future._asyncComplete.<anonymous closure> (dart:async/future_impl.dart:393)
#14     _asyncRunCallbackLoop (dart:async/schedule_microtask.dart:23)
#15     _asyncRunCallback (dart:async/schedule_microtask.dart:32)
#16     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:126)


Unhandled exception:
The null object does not have a method 'expand'.

NoSuchMethodError: method not found: 'expand'
Receiver: null
Arguments: [Closure: (int) => dynamic]
#0      _rootHandleUncaughtError.<anonymous closure> (dart:async/zone.dart:826)
#1      _asyncRunCallbackLoop (dart:async/schedule_microtask.dart:23)
#2      _asyncRunCallback (dart:async/schedule_microtask.dart:32)
#3      _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:126)