How to fix LateInitializationError about non-nullable variables? - flutter

Question

I was studying with an AWS tutorial however, that tutorial is not null safety.
I'v tried to convert it, but is showing:
LateInitializationError: Field '_credentials#26120019' has not been initialized.
I think that "late" modifier is not initializing the variables when I try to get the values in verifyCode
Pleease, how can I fix the code below
enum AuthFlowStatus {login, signUp, verification, session}
class AuthState {
final AuthFlowStatus? authFlowStatus;
AuthState({this.authFlowStatus});
}
class AuthService {
final authStateController = StreamController<AuthState>();
late AuthCredentials _credentials;
void showSignUp() {
final state = AuthState(authFlowStatus: AuthFlowStatus.signUp);
authStateController.add(state);
}
void showLogin() {
final state = AuthState(authFlowStatus: AuthFlowStatus.login);
authStateController.add(state);
}
void loginWithCredentials(AuthCredentials credentials) async {
try {
final result = await Amplify.Auth.signIn(
username: credentials.username, password: credentials.password,
);
if (result.isSignedIn) {
final state = AuthState(authFlowStatus: AuthFlowStatus.session);
authStateController.add(state);
} else {
print('User could not be signed in');
}
} on AuthException catch (authError) {
print('Could not login - ${authError}');
}
}
void signUpWithCredentials(SignUpCredentials credentials) async {
try {
Map<CognitoUserAttributeKey, String> userAttributes = {
CognitoUserAttributeKey.email: credentials.email,
};
final result = await Amplify.Auth.signUp(
username: credentials.username,
password: credentials.password,
options: CognitoSignUpOptions(
userAttributes: userAttributes
),
);
if (result.isSignUpComplete) {
loginWithCredentials(credentials);
} else {
this._credentials = credentials;
}
final state = AuthState(authFlowStatus: AuthFlowStatus.verification);
authStateController.add(state);
} on AmplifyException catch (authError) {
print('Failed ro sign up - ${authError}');
}
}
void verifyCode(String verificationCode) async {
try {
final result = await Amplify.Auth.confirmSignUp(
username: _credentials.username,
confirmationCode: verificationCode,
);
if (result.isSignUpComplete) {
loginWithCredentials(_credentials);
} else {
//not implemented yet
}
} on AuthException catch (authError) {
print('Could not verify code - ${authError}');
}
}
}