Snackbar on login failure not showing cause of credentials never being null , how to get around this

Snackbar on login failure not showing cause of credentials never being null , how to get around this also loading will always be true in case of wrong email and password.Caused by null safety

ElevatedButton(
                          onPressed: () {
                            if (_formKey.currentState!.validate()) {
                              setState(() {
                                _loading = true;
                              });
                              _authData
                                  .loginCreatorByMail(email, password)
                                  .then((credential) {
                                if (credential.user!.uid != null) {
                                  setState(() {
                                    _loading = false;
                                  });
                                  Navigator.pushReplacementNamed(
                                      context, UserHomeScreen.id);
                                } else {
                                  setState(() {
                                    _loading = false;
                                  });
                                  ScaffoldMessenger.of(context)
                                      .showSnackBar(SnackBar(
                                          content: Text(
                                              _authData
                                                  .loginCreatorByMailError
                                                  .toString(),
                                              textAlign: TextAlign.center),
                                          backgroundColor: Colors.pink));
                                }
                              });
                            }
                          },
                          child: _loading
                              ? const SpinKitFadingCircle(
                                  color: Colors.pink)
                              : const Text(
                                  'Login',
                                  style: TextStyle(fontSize: 18),
                                ),
                          style: ElevatedButton.styleFrom(
                            primary: const Color.fromRGBO(73, 12, 97, 1),
                              shape: RoundedRectangleBorder(
                              borderRadius: BorderRadius.circular(12.0),
                            ),
                          ),
                        ),
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