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Question

Android Studio 3.6

 @override
  Widget build(BuildContext context) {
    SystemChrome.setSystemUIOverlayStyle(SystemUiOverlayStyle(
        statusBarColor: new Color(Constants.COLOR_PRIMARY_DARK)));
    return new Scaffold(
        body: SafeArea(
            child: SingleChildScrollView(
                child: new Container(
                    margin: const EdgeInsets.only(
                        left: Constants.DEFAULT_MARGIN,
                        right: Constants.DEFAULT_MARGIN,
                        bottom: Constants.DEFAULT_MARGIN),
                    child: new Column(children: [
                      new Padding(
                          padding:
                              EdgeInsets.only(top: Constants.DEFAULT_MARGIN),
                          child: _createScanCheckContainer(context)),
                      ])))));
  }

  Widget _createScanCheckContainer(BuildContext context) {
    return new GestureDetector(
        onTap: () {
          _logger
              .d("_createScanCheckContainer: onTap -> forward_to_scan_screen");
          // only for test
          _showBottomSheetNotification(context);
        },
        child: new Container(
            height: 56.0,
            decoration: new BoxDecoration(
                borderRadius: BorderRadius.all(
                    Radius.circular(Constants.ROUNDED_CORNER_RADIUS)),
                color: Color(Constants.COLOR_PRIMARY),
                boxShadow: [_createBoxShadow()]),
            child: Stack(children: [
              new Positioned(
                left: Constants.DEFAULT_MARGIN,
                top: Constants.DEFAULT_MARGIN,
                bottom: Constants.DEFAULT_MARGIN,
                child: new Image.asset('assets/images/ic_scan_ticket.png'),
              ),
              new Expanded(
                  child: Align(
                alignment: Alignment.center,
                child: Text("Scan receipt",
                    style: TextStyle(fontSize: 19.0, color: Colors.white)),
              ))
            ])));
  }

  void _showBottomSheetNotification(BuildContext context) {
    _logger.d("_showBottomSheetNotification_start");
    showBottomSheet(
        context: context,
        builder: (context) {
          return new Container(
            height: 135.0,
            color: Colors.orange,
            child: new Text("Test message",
                style: TextStyle(color: Colors.white)),
          );
        });
  }

When I tap the message " _showBottomSheetNotification_start " is show in logcat but bottomSheet not show.

error:

════════ Exception caught by gesture ═══════════════════════════════════════════════════════════════
The following assertion was thrown while handling a gesture:
No Scaffold widget found.

MainScreen widgets require a Scaffold widget ancestor.
The specific widget that could not find a Scaffold ancestor was: MainScreen
The ancestors of this widget were: 
  : MaterialApp
    state: _MaterialAppState#2c9e7
  : MyApp
  ...

Typically, the Scaffold widget is introduced by the MaterialApp or WidgetsApp widget at the top of your application widget tree.

When the exception was thrown, this was the stack: 
#0      debugCheckHasScaffold.<anonymous closure> (package:flutter/src/material/debug.dart:112:7)
#1      debugCheckHasScaffold (package:flutter/src/material/debug.dart:123:4)
#2      showBottomSheet (package:flutter/src/material/bottom_sheet.dart:721:10)
#3      MainScreen._showBottomSheetNotification (package:flutter_sample/screens/main_screen.dart:375:5)
#4      MainScreen._createScanCheckContainer.<anonymous closure> (package:flutter_sample/screens/main_screen.dart:183:11)
...
Handler: "onTap"

Answer 1

Try wrapping your MainScreen widget inside MaterialApp as it's home property. Example:

void main() => runApp(new MaterialApp(home: new MainScreen()));

class MainScreen extends StatelessWidget {

  // some cool codes ;-) 

}

Solution Reference: See this issue