How To Add Checked Enter Value Number Or Not In Flutter

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
        home: Scaffold(
            appBar: AppBar(
              centerTitle: true,
                backgroundColor: Colors.amber,
                title: Text('Check Value Is Number Or Not',style: TextStyle(color: Colors.black),
                )),
            body: Center(child: TextFieldClass())));
  }
}

class TextFieldClass extends StatefulWidget {
  TextFieldClassState createState() => TextFieldClassState();
}

class TextFieldClassState extends State<TextFieldClass> {
  final textFieldHolder = TextEditingController();

  String value = '';
  String output = 'Not Checked';

  checkTextInputData() {
    setState(() {
      value = textFieldHolder.text;
    });

    print(_isNumeric(value));

    if (_isNumeric(value) == true) {
      setState(() {
        output = 'Value is Number';
      });
    } else {
      setState(() {
        output = 'Value is Not Number';
      });
    }
  }

  bool _isNumeric(String result) {
    if (result == null) {
      return false;
    }
    return double.tryParse(result) != null;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              Container(
                  width: 280,
                  padding: EdgeInsets.all(10.0),
                  child: TextField(
                    controller: textFieldHolder,
                    autocorrect: true,
                    decoration: InputDecoration(
                        hintText: 'Enter Some Text'),
                  )),
              RaisedButton(
                onPressed: checkTextInputData,
                color: Colors.amber,
                textColor: Colors.black,
                padding: EdgeInsets.fromLTRB(10, 10, 10, 10),
                child: Text('Click Here Value Is Number Or Not'),
              ),
              Padding(
                  padding: EdgeInsets.all(8.0),
                  child: Text("$output", style: TextStyle(fontSize: 20)))
            ],
          ),
        ));
  }
}