title: Failed assertion: line 190 pos 15: 'items!= null': is not true
body: <p>I am trying to build a Bottom Curved navigation bar in the flutter, I have tried many examples but nothing worked for me. Even I used the same example as it is on their official website but nothing happened.
  I am trying to build a Bottom Curved navigation bar in the flutter, I have tried many examples but nothing worked for me. Even I used the same example as it is on their official website but nothing happened.</p>
  
  <pre><code>import 'package:flutter/material.dart';
  import 'package:curved_navigation_bar/curved_navigation_bar.dart';
  
  void main() =&gt; runApp(MaterialApp(home: BottomNavBar()));
  
  class BottomNavBar extends StatefulWidget {
    @override
    _BottomNavBarState createState() =&gt; _BottomNavBarState();
  }
  
  class _BottomNavBarState extends State&lt;BottomNavBar&gt; {
    int _page = 0;
    GlobalKey _bottomNavigationKey = GlobalKey();
  
    @override
    Widget build(BuildContext context) {
      return Scaffold(
          bottomNavigationBar: CurvedNavigationBar(
            key: _bottomNavigationKey,
            index: 0,
            height: 50.0,
            items: &lt;Widget&gt;[
              Icon(Icons.add, size: 30),
              Icon(Icons.list, size: 30),
              Icon(Icons.compare_arrows, size: 30),
              Icon(Icons.call_split, size: 30),
              Icon(Icons.perm_identity, size: 30),
            ],
            color: Colors.white,
            buttonBackgroundColor: Colors.white,
            backgroundColor: Colors.blueAccent,
            animationCurve: Curves.easeInOut,
            animationDuration: Duration(milliseconds: 600),
            onTap: (index) {
              setState(() {
                _page = index;
              });
            },
          ),
          body: Container(
            color: Colors.blueAccent,
            child: Center(
              child: Column(
                children: &lt;Widget&gt;[
                  Text(_page.toString(), textScaleFactor: 10.0),
                  RaisedButton(
                    child: Text('Go To Page of index 1'),
                    onPressed: () {
                      final CurvedNavigationBarState navBarState =
                          _bottomNavigationKey.currentState;
                      navBarState.setPage(1);
                    },
                  )
                ],
              ),
            ),
          ));
    }
  }
  </code></pre>