title: Future inside MaterialPageRoute builder returns null
body: <p>Need to query server for an object before pushing the ProfileWidget route. </p>
  
  <p>Not sure if error message has anything to do with the Future call inside the MaterialPageRoute's builder (see routes function)?</p>
  
  <p><strong><em>ERROR</em></strong></p>
  
  <blockquote>
    <p>flutter: Another exception was thrown: The builder for route "null" returned null.</p>
  </blockquote>
  
  <pre><code>..
         @override
            Widget build(BuildContext context) {
              return new MaterialApp(
                onGenerateRoute: routes,
                debugShowCheckedModeBanner: false,
                theme: new ThemeData(
                    primarySwatch: Colors.blueGrey,
                    textTheme: TextTheme(
  
                    )
                ),
                home: ...,
              )
              ;
            }
  
            Route routes(RouteSettings settings) {
              if (settings.name.startsWith("/profile")) {
                String username = settings.name.replaceFirst("/profile/", '');
                return MaterialPageRoute(
                    builder: (context) {
                      api.fetchProfile(username)
                          .then((profile) {
                        return ProfileWidget(profile);
                      });
                    }
  
                );
              }
            }
  </code></pre>