Fitbit Api Oauth Issue

[Sheing]

I am trying to connect to fitbit oauth, here is my code:

Map<String, String> customParameters = {"scope": "activity"};
final OAuth flutterOAuth = new FlutterOAuth(new Config(
"https://www.fitbit.com/oauth2/authorize",
"https://api.fitbit.com/oauth2/token",
"######",
"################",
"http://localhost:8080",
"code",
parameters:customParameters,
contentType: "application/x-www-form-urlencoded"));

But i keep getting a null return after I logged in and allowed access to app.

Error:
I/flutter ( 6554): null
E/flutter ( 6554): [ERROR:topaz/lib/tonic/logging/dart_error.cc(16)] Unhandled exception:
E/flutter ( 6554): 'package:flutter/src/widgets/text.dart': Failed assertion: line 212 pos 15: 'data != null': is not true.
E/flutter ( 6554): #0 _AssertionError._doThrowNew (dart:core-patch/dart:core/errors_patch.dart:37)
E/flutter ( 6554): #1 _AssertionError._throwNew (dart:core-patch/dart:core/errors_patch.dart:33)
E/flutter ( 6554): #2 new Text (package:flutter/src/widgets/text.dart:212:15)
E/flutter ( 6554): #3 MainState.authorise (/data/user/0/co.joebirch.tide/cache/exampleFJXSCW/example/lib/main.dart:71:20)
E/flutter ( 6554):
E/flutter ( 6554): #4 MainState.build. (/data/user/0/co.joebirch.tide/cache/exampleFJXSCW/example/lib/main.dart:47:36)
E/flutter ( 6554): #5 _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:478:14)
E/flutter ( 6554): #6 _InkResponseState.build. (package:flutter/src/material/ink_well.dart:530:30)