title: The getter 'value' was called on null
body: <p>I am trying to create this font size animation in flutter but I am receiving the following error: The getter 'value' was called on null.</p>
  
  <p>It seems weird because I have another dart file with similar code and it works perfectly.</p>
  
  <p><strong><em>Any help would be strongly appreciated.</em></strong>    </p>
  
  <pre><code>Animation&lt;double&gt; _fontSizeAnimation;
  AnimationController _fontSizeAnimationController;
  </code></pre>
  
  <p>initState function():</p>
  
  <pre><code>super.initState();
  _fontSizeAnimationController = new AnimationController(
      duration: new Duration(milliseconds: 500), vsync: this);
  _fontSizeAnimation = new CurvedAnimation(
      parent: _fontSizeAnimationController, curve: Curves.bounceOut);
  _fontSizeAnimation.addListener(() =&gt; this.setState(() {}));
  _fontSizeAnimationController.forward();
  </code></pre>
  
  <p>Material widget</p>
  
  <pre><code>  return new Material(
    color: Colors.blueAccent,
    child: new InkWell(
      onTap: () =&gt; print("We tapped the page!"),
      child: new Column(
        mainAxisAlignment: MainAxisAlignment.center,
        children: &lt;Widget&gt;[
          new Text("Let's Quizzz",
              style: new TextStyle(
                  color: Colors.white,
                  fontSize: _fontSizeAnimation.value * 15,
                  fontWeight: FontWeight.bold)),
          new Text("Tap to start!",
              style: new TextStyle(
                  color: Colors.white,
                  fontSize: 20.0,
                  fontWeight: FontWeight.bold))
        ],
      ),
    ),
  );
  </code></pre>