No Scaffold widget found : Getting exception while opening Bottom Dialog sheet

The problem is that the context used to show the BottomSheet is not the context of the Scaffold. You can solve this issue by using GlobalKey or wrap your GestureDetector in a Builder widget so that it gives you the context that contains a reference of the Scaffold.

Here is a example using GlobalKey with the state of the Scaffold:

// created the ScaffoldState key    
final scaffoldState = GlobalKey<ScaffoldState>();
    
    class MyWidget extends StatelessWidget {
      void _showSheet() {
        // Show BottomSheet here using the Scaffold state instead ot«f the Scaffold context
        scaffoldState.currentState
            .showBottomSheet((context) => Container(color: Colors.red));
      }
    
      @override
      Widget build(BuildContext context) {
        return Scaffold(
            key: scaffoldState,
            appBar: AppBar(
              title: Text("Calendar"),
            ),
            body: SafeArea(child: GestureDetector(onTap: () {
              //getting exception here
              _showSheet();
            })));
      }
    }