title: The argument type 'GlobalKey<NavigatorState>?' can't be assigned to the parameter type'GlobalKey<NavigatorState>'
body: <p>I am getting this error:</p>
  <pre><code>The argument type 'GlobalKey&lt;NavigatorState&gt;?' can't be assigned to
  the parameter type'GlobalKey&lt;NavigatorState&gt;'
  
  .dart(argument_type_not_assignable)
  Map&lt;BottomNavItem, GlobalKey&lt;NavigatorState&gt;&gt; navigatorKeys
  </code></pre>
  <p>I do believe that I am passing the correct thing. But where does this question mark at the end shows up?</p>
  <pre><code> Widget _buildOffStageNavigatorItem(
      BottomNavItem currentItem,
      bool isSelected,
    ) {
      return Offstage(
        offstage: !isSelected,
        child: TabNavigator(
          item: currentItem,
          navigatorKey: navigatorKeys[currentItem],
        ),
      );
    }
  
  final Map&lt;BottomNavItem, GlobalKey&lt;NavigatorState&gt;&gt; navigatorKeys = {
      BottomNavItem.home: GlobalKey&lt;NavigatorState&gt;(),
      BottomNavItem.scorecard: GlobalKey&lt;NavigatorState&gt;(),
      BottomNavItem.rewards: GlobalKey&lt;NavigatorState&gt;(),
      BottomNavItem.orders: GlobalKey&lt;NavigatorState&gt;(),
    };
  </code></pre>
  <p>pls help</p>