GQLError shouldn't require 'locations'

[S7evinK]

I'm currently trying to run a mutation, which doesn't return locations, if there was an error.
Unfortunately, it seems like locations is required in case there was an error.

NoSuchMethodError: The method 'map' was called on null.
Receiver: null
Tried calling: map(Closure: (dynamic) => Location)
#0      Object.noSuchMethod (dart:core/runtime/libobject_patch.dart:46:5)
#1      new GQLError.fromJSON (package:graphql_client/src/exceptions.dart:39:33)
#2      new GQLException.<anonymous closure> (package:graphql_client/src/exceptions.dart:61:44)
#3      MappedListIterable.elementAt (dart:_internal/iterable.dart:414:29)
#4      ListIterator.moveNext (dart:_internal/iterable.dart:343:26)
#5      new List.from (dart:core/runtime/libarray_patch.dart:51:29)
#6      new GQLException (package:graphql_client/src/exceptions.dart:61:7)
#7      GQLClient._parseResponse (package:graphql_client/src/client.dart:109:17)
#8      GQLClient.execute (package:graphql_client/src/client.dart:65:20)

Query:

mutation($username: String!, $password: String!) {
  createToken (username: $username, password: $password) { 
  	token
  }
}

Response:

{
  "data": {
    "createToken": null
  },
  "errors": [
    {
      "message": "Username and password required.",
      "path": [
        "createToken"
      ]
    }
  ]
}

Successfull request work as expected.
I've tried to modify exceptions.dart, but without luck. (Just started with Dart a few days ago.)