title: Common Flutter Error (missing required argument)
body: <p>I am new to flutter and I was trying to test some liblaries.
  But unfortunately when I run this code, I get error:</p>
  <p><em>error mesage:</em></p>
  <pre><code>error G4A9793AD: The value 'null' can't be assigned to the parameter type 'Key' because 'Key' is not nullable.
  </code></pre>
  <p><a href="https://i.stack.imgur.com/gEqfD.png" rel="nofollow noreferrer">error image url</a></p>
  <pre><code>...some code...
      Widget build(BuildContext context) {
          return MaterialApp(
            debugShowCheckedModeBanner: false,
            home: MainPage(title: 'Flutter Convex BottomBar Sample'),
          );
        }
      }
   
  class MainPage extends StatefulWidget {
    MainPage({required Key key, required this.title}) : super(key: key);
  ...some code...
  </code></pre>