Show/Hide data inside the dialog box in flutter - flutter

Question

I have a dialog box, which has a TextField(), and a Button. On press of it, I'm calling my API, but before that, I need to put a check on the text. Since everything works fine, I was wondering to find some way out to show the error text inside the dialog box only.
After all the research and work, the dialog box is not recreating itself hence no message is showing even if the boolean case becomes true.
CODE:
class ListViewElements extends StatefulWidget {
#override
_ListViewElementsState createState() => _ListViewElementsState();
}
class _ListViewElementsState extends State<ListViewElements> {
bool isError = false;
bool errorText = false;
final TextEditingController _code = new TextEditingController();
onSubmit(BuildContext context){
if(this._code.text.isEmpty){
setState(
(){
isError = true;
errorText = 'Empty space';
}
);
}
}
void registerDialog(BuildContext context){
showDialog(
context: context,
barrierDismissible: false,
builder: (BuildContext context){
return AlertDialog(
content: Column(
children: <Widget>[
Container(
margin: EdgeInsets.fromLTRB(40.0, 0.0, 40.0, 10.0),
child: TextField(
textAlign: TextAlign.center,
controller: this._code,
maxLength: 4,
textInputAction: TextInputAction.done,
cursorColor: Theme.of(context).primaryColor,
keyboardType: TextInputType.number,
decoration: InputDecoration(
hintText: 'Enter here',
counterText: '',
)
)
),
isError == true ? Text(this.erroText) : Container()
]
),
actions: <Widget>[
FlatButton(
child: new Text("Done", style: TextStyle(color: Theme.of(context).primaryColor, fontSize: 17.0)),
onPressed: () {onSubmit(context);}
)
]
);
}
);
I thought of showing a SnackBar(), but for dialog box, Scaffold.of() is not working out, since the builder is creating the new instance of the box, hence cannot recall. Any help would be appreciated. Thanks :)