I built the app and the message so error like this: Receiver: null

[Lorngdy68]

E/flutter (11754): [ERROR:flutter/lib/ui/ui_dart_state.cc(186)] Unhandled Exception: NoSuchMethodError: The method '[]' was called on null.
E/flutter (11754): Receiver: null
E/flutter (11754): Tried calling:
E/flutter (11754): #0 Object.noSuchMethod (dart:core-patch/object_patch.dart:54:5)
E/flutter (11754): #1 new Users.fromMap (package:skypeclone/models/users.dart:33:23)
E/flutter (11754): #2 FirebaseMethods.getUserDetails (package:skypeclone/resources/firebasemethods.dart:31:18)
E/flutter (11754):
E/flutter (11754): #3 UserProvider.refreshUser (package:skypeclone/provider/userprovider.dart:12:19)
E/flutter (11754):
E/flutter (11754):