title: Flutter Calculator Issue
body: <p>How can I fix that issue in Flutter, and what is the same thing of id in Native Android?
  I've made a calculator but the result doesn't appear in the Text widget.
  After pressing on one of the arithmetic operators </p>
  
  <blockquote>
    <p>it should show the result on the Text "null",</p>
  </blockquote>
  
  <p>but nothing happened.</p>
  
  <p>I've attached an image for the problem<a href="https://i.stack.imgur.com/jRYfa.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jRYfa.jpg" alt="Flutter Image App"></a>
  This is the code</p>
  
  <pre><code>import 'package:flutter/material.dart';
  import 'dart:math';
  
  void main() {
  runApp(MaterialApp(home: Calculator()));
  }
  
  class Calculator extends StatefulWidget {
  @override
  _CalculatorState createState() =&gt; _CalculatorState();
  }
  
  class _CalculatorState extends State&lt;Calculator&gt; {
  var theResult, firstNum, secondNum;
  void divideTheNums() {
  setState(() {
  theResult = firstNum / secondNum;
  });
  }
  
  void multiplyTheNums() {
  setState(() {
  theResult = firstNum * secondNum;
  });
  }
  
  void addTheNums() {
  setState(() {
  theResult = firstNum + secondNum;
  });
  }
  
  void subtractTheNums() {
  setState(() {
  theResult = firstNum - secondNum;
  });
  }
  
  Widget build(BuildContext context) {
  return new Scaffold(
  resizeToAvoidBottomPadding: false,
  appBar: AppBar(
  backgroundColor: Colors.redAccent,
  centerTitle: true,
  title: Text(
  "Calculator",
  ),
  ),
  body: Padding(
  padding: const EdgeInsets.all(16.0),
  child: Container(
  child: Center(
  child: Column(
  mainAxisAlignment: MainAxisAlignment.center,
  children: &lt;Widget&gt;[
  Row(
  mainAxisAlignment: MainAxisAlignment.center,
  children: &lt;Widget&gt;[
  Text(
  "OutPut:",
  style: TextStyle(
  fontSize: 25.0, fontWeight: FontWeight.w700),
  ),
  Text(theResult.toString()),
  Container(
  margin: EdgeInsets.only(bottom: 30.0), child: Text(""))
  ],
  ),
  Container(
  margin: EdgeInsets.only(bottom: 20.0),
  child: TextField(
  controller: TextEditingController(),
  decoration: InputDecoration(
  hintText: "Enter First Number:",
  border: OutlineInputBorder()),
  ),
  ),
  TextField(
  controller: TextEditingController(),
  decoration: InputDecoration(
  hintText: "Enter Second Number:",
  border: OutlineInputBorder()),
  ), Row(mainAxisAlignment: MainAxisAlignment.spaceEvenly,
  children: &lt;Widget&gt;[
  Container(
  margin: EdgeInsets.only(right: 20.0),
  child: RaisedButton(
  color: Colors.redAccent,
  onPressed: divideTheNums,
  child: Text(
  "/",
  style: TextStyle(color: Colors.white, fontSize: 23.0),
  ),
  ),
  ),
  Container(
  margin: EdgeInsets.only(),
  child: RaisedButton(
  color: Colors.redAccent,
  onPressed: multiplyTheNums,
  child: Text(
  "*",
  style: TextStyle(color: Colors.white, fontSize: 23.0),
  ),
  ),
  )
  ],
  ),
  Row(
  mainAxisAlignment: MainAxisAlignment.spaceEvenly,
  children: &lt;Widget&gt;[
  Container(
  margin: EdgeInsets.only(right: 20.0, top: 20.0),
  child: RaisedButton(
  color: Colors.redAccent,
  highlightColor: Colors.white,
  onPressed: subtractTheNums,
  child: Text(
  "-",
  style: TextStyle(color: Colors.white, fontSize: 23.0),
  ),
  ),
  ),
  Container(
  margin: EdgeInsets.only(top: 20.0),
  child: RaisedButton(
  color: Colors.redAccent,
  onPressed: addTheNums,
  child: Text(
  "+",
  style: TextStyle(color: Colors.white, fontSize: 23.0),
  ),
  ),
  )
  ],
  )
  ],
  ),
  ),
  ),
  ),
  );
  }
  }
  </code></pre>