login method Null safety
Implementation
static Future<NCMBUser> login(String userName, String password) async {
Map _fields = {'userName': userName, 'password': password};
NCMBRequest r = new NCMBRequest();
Map response =
await r.exec('GET', 'users', queries: _fields, path: 'login');
var user = NCMBUser();
user.setLoginResponse(response);
return user;
}