title: globalkey formstate return false when validation
body: <p>i'm new in Flutter and i am trying to create sign-in form validation and use this <a href="https://gist.github.com/diegoveloper/1cd23e79a31d0c18a67424f0cbdfd7ad" rel="nofollow noreferrer">FadeIndexedStack</a>. but always get false when i call <em>formKey.currentState.validate()</em>. formKey is global key form state</p>
  <ol>
  <li>the form looks like this:
  <a href="https://i.stack.imgur.com/8606T.png" rel="nofollow noreferrer">form stucture</a>.</li>
  <li>the textformfield is like this:</li>
  </ol>
  <pre><code>Container( //container textformfield (inside fadeindexedstack)
   //rest code
  
   TextFormField(
    validator: (val) =&gt; val.length &lt; 3 ? ' too short.' : null,
    onChanged: (String value) {
     username = value;
    },
   ),
   TextFormField(
    validator: (val) =&gt; val.length &lt; 3 ? ' too short.' : null,
    onChanged: (String value) {
     password = value;
    },
   ),
  
   //rest code
  )
  </code></pre>
  <ol start="3">
  <li>the button which returns validate:</li>
  </ol>
  <pre><code>FlatButton( //outside fadeindexedstack
   onPressed: () {
     print(formKey.currentState.validate());
   }
  </code></pre>
  <p>how do i get <em>true</em> value for validation?</p>