title: The class 'SimpleFoldingCell' doesn't have a default constructor
body: <p><a href="https://github.com/flutter-devs/flutter_folding_cell_demo/blob/master/lib/demo_screen.dart" rel="nofollow noreferrer">https://github.com/flutter-devs/flutter_folding_cell_demo/blob/master/lib/demo_screen.dart</a></p>
  <p>I'm trying to follow the code in the link above.
  But in the SimpleFoldingCell part, an error named 'The class 'SimpleFoldingCell' doesn't have a default constructor.' occurs.
  Could
  Is there a way to resolve this error in Dart?</p>
  <pre><code>class Notific extends StatefulWidget{
    @override
    _State createState() =&gt; _State();
  }
  
  class _State extends State&lt;Notific&gt;{
    late List&lt;TechnologyModel&gt; _technologyList;
  
    @override
    void initState() {
      super.initState();
      _technologyList = [
        TechnologyModel(title: &quot;Application Development&quot;,),
        TechnologyModel(title: &quot;Research &amp; Development&quot;,),
        TechnologyModel(title: &quot;Big Data &amp; Analytics&quot;,),
        TechnologyModel(title: &quot;Support Services&quot;,),
        TechnologyModel(title: &quot;QA &amp; Software Testing&quot;,),
      ];
  
    }
    @override
    Widget build(BuildContext context) {
      return Scaffold(
        appBar: AppBar(
          automaticallyImplyLeading: false,
          backgroundColor: Colors.pink[200],
          title: Text(&quot;Folding Cell Demo&quot;,
            style: TextStyle(color: Colors.white),),
        ),
        body: Container(
          child: ListView.builder(
            itemCount: _technologyList.length,
            itemBuilder: (context, index) {
              return SimpleFoldingCell(
                frontWidget: _buildFrontWidget(index),
                innerTopWidget: _buildInnerWidget(index),
                innerBottomWidget: _buildInnerBottomWidget(),
                cellSize: Size(MediaQuery.of(context).size.width, 125),
                padding: EdgeInsets.all(15),
                animationDuration: Duration(milliseconds: 200),
                borderRadius: 10,
                onOpen: () =&gt; print('$index cell opened'),
                onClose: () =&gt; print('$index cell closed'),
              );
            },
          ),
        ),
      );
    }
  
    Widget _buildFrontWidget(int index) {
      return Builder(
        builder: (BuildContext context) {
          return Container(
            color:  Colors.cyan[100],
            alignment: Alignment.bottomCenter,
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: [
                Text(
                  &quot;Aeologic Technology&quot;,
                  style: TextStyle(
                    fontSize:20.0,
                    color: Colors.black,
                  ),
                ),
                SizedBox(height: 10,),
                FlatButton(
                  onPressed: () {
                    final foldingCellState = context
                        .findAncestorStateOfType&lt;SimpleFoldingCellState&gt;();
                    foldingCellState?.toggleFold();
                  },
                  child: Text(
                    &quot;OPEN&quot;,
                  ),
                  textColor: Colors.white,
                  color: Colors.indigoAccent[100],
                  splashColor: Colors.white.withOpacity(0.5),
                ),
              ],
            ),
          );
        },
      );
    }
  
    Widget _buildInnerBottomWidget() {
      return Builder(
          builder: (context) {
            return Container(
              color: Colors.blueGrey[50],
              alignment: Alignment.bottomCenter,
              child: Padding(
                padding: EdgeInsets.only(bottom: 10),
                child: FlatButton(
                  onPressed: () {
                    final foldingCellState = context
                        .findAncestorStateOfType&lt;SimpleFoldingCellState&gt;();
                    foldingCellState?.toggleFold();
                  },
                  child: Text(
                    &quot;Close&quot;,
                  ),
                  textColor: Colors.white,
                  color: Colors.redAccent[100],
                  splashColor: Colors.white.withOpacity(0.5),
                ),
              ),
            );
          }
      );
    }
  
    Widget _buildInnerWidget(int index) {
      return Builder(
        builder: (context) {
          return Container(
            color: Colors.pink[100],
            padding: EdgeInsets.only(top: 10),
            child: Align(
              alignment: Alignment.center,
              child: Text(
                _technologyList[index].title,
                style: TextStyle(
                  fontSize:20.0,
                  color: Colors.black,
                ),
              ),
            ),
          );
        },
      );
    }
  }
  </code></pre>
  <pre><code>class TechnologyModel {
    String title;
  
    TechnologyModel({
      required this.title,
    });
  }
  </code></pre>