RangeError (start): Invalid value: Only valid value is 0: -16 - FLUTTER - flutter

Question

I am trying to encrypt a password locally, it is created or changed when the user wishes.
But when I try to change it from the user interface I get this error:
Unhandled Exception: RangeError (start): Invalid value: Only valid value is 0: -16.
although the app still works.
This is the class code:
class SettingsPage extends StatefulWidget {
#override
_State createState() => _State();
static String routeName = '/settings';
static bool switched = false;
static String inputPassword = '0000';
}
class _State extends State<SettingsPage> {
static final key = encrypt.Key.fromLength(32);
static final iv = encrypt.IV.fromLength(1);
static final encrypter = encrypt.Encrypter(encrypt.AES(key));
static encryptAES(text) {
final encrypted = encrypter.encrypt(text, iv: iv);
print(encrypted.bytes);
print(encrypted.base16);
print(encrypted.base64);
return encrypted;
}
static decryptAES(text) {
return encrypter.decrypt(text, iv: iv);
}
TextEditingController tec = TextEditingController();
var encryptedText, plainText;
Future<void> localAuth(BuildContext context) async {
final localAuth = LocalAuthentication();
final didAuthenticate = await localAuth.authenticateWithBiometrics(
localizedReason: 'Please authenticate');
if (didAuthenticate) {
Navigator.pop(context);
}
}
Future<SharedPreferences> _prefs = SharedPreferences.getInstance();
late SharedPreferences prefs;
getSwitchValues() async {
SettingsPage.switched = await getSwitchState();
setState(() {});
}
getPassValues() async{
prefs = await _prefs;
setState(() {
prefs.containsKey('savedPass')? prefs.getString('savedPass'): "";
setState(() {
});
});
}
#override
void initState() {
super.initState();
getSwitchValues();
getPassValues();
}
Future<bool> getSwitchState() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
SettingsPage.switched = prefs.getBool("switched")!;
print(SettingsPage.switched);
return SettingsPage.switched;
}
Future<bool> saveSwitchState(bool value) async {
SharedPreferences prefs = await SharedPreferences.getInstance();
prefs.setBool("switched", value);
print('Switch Value saved $value');
return prefs.setBool("switched", value);
}
#override
Widget build(BuildContext context) {
return Container(
padding: EdgeInsets.all(paddingSizeLarge),
child:
Column(
children: [
Text('SETTINGS',
style: GoogleFonts.encodeSansSemiCondensed(fontSize: 35.0, fontWeight: FontWeight.w700,color: Palette.secondaryTextColor),
textAlign: TextAlign.center,
),
VerticalSpacing(of: marginSizeDefault),
Container(
child:
ListView(
shrinkWrap: true,
padding: EdgeInsets.all(paddingSizeDefault),
children: [
Row(
children:[
Text('Activate Biometrics : ',
style: GoogleFonts.encodeSansSemiCondensed(fontSize: 20.0, fontWeight: FontWeight.w700,color: Palette.textColor),),
HorizontalSpacing(of: marginSizeXXLarge),
Switch(
value: SettingsPage.switched,
onChanged: (value) {
setState(() {
SettingsPage.switched = value;
saveSwitchState(SettingsPage.switched);
});
screenLock(context: context, correctString: '1234',
canCancel: true,
customizedButtonTap: () async {
await localAuth(context);
},
didOpened: () async {
await localAuth(context);
},);
},
activeTrackColor: Colors.blue,
activeColor: Colors.blue,
),
]),
VerticalSpacing(of: marginSizeLarge),
/* Row(
children:[
Text('Biometrics Settings : ',
style: GoogleFonts.encodeSansSemiCondensed(fontSize: 20.0, fontWeight: FontWeight.w700,color: Palette.textColor),),
HorizontalSpacing(of: marginSizeXXLarge),
IconButton(onPressed: (){ print('bio set');} , icon: const Icon(FeatherIcons.arrowRight, color: Colors.black,))
]),
VerticalSpacing(of: marginSizeLarge),*/
Row(
children:[
Text('Create/Change Password : ',
style: GoogleFonts.encodeSansSemiCondensed(fontSize: 20.0, fontWeight: FontWeight.w700,color: Palette.textColor),),
IconButton( icon: const Icon(FeatherIcons.arrowRight, color: Colors.black,),
onPressed: () async {
final result = await
showOkCancelAlertDialog(
context: context,
title: 'Change or Create a new Password',
message:
'This will remove your existing Password or create a new one',
);
if (result == OkCancelResult.ok) {
final input = (await showTextInputDialog(
textFields: [DialogTextField(keyboardType: TextInputType.numberWithOptions(decimal: true),
),
],
context: context,
title: 'Change or Create a new Password',
message: 'enter your new password',
))!.join();
if (input != null && input.length > 0 && input.length == 4 ) {
context.read<SettingsBloc>().changePassword(input);
SettingsPage.inputPassword = input;
SettingsPage.inputPassword = tec.text;
setState(() {
encryptedText = encryptAES(SettingsPage.inputPassword);
print("PROVA ENCRYPTED TEXT "+encryptedText);
});
prefs.setString('savedPass', encryptedText);
encryptedText = decryptAES(encryptedText);
print("PROVA TESTO DECRIPTATO " + encryptedText);
}
}
},
),
]),
VerticalSpacing(of: 50),
RoundedButton(text:'Log Out',),
],
),
decoration: new BoxDecoration(
color: Palette.primaryLightColor,
borderRadius: BorderRadius.circular(20),
boxShadow: [
BoxShadow(color: Palette.backgroundColor,
),
],
),
),
]
),
);
}
}
both texts are not written to the prompt with the print function. there must be some problem within the conditions and in addition the error I get when changing the password, please if anyone knows how to help me come forward thanks.