Flutter-restrict-device-orientation.text

Way 01:
---------------------------------------------------------
import 'package:flutter/services.dart';

void main() {
  SystemChrome.setPreferredOrientations(
          [DeviceOrientation.portraitUp,DeviceOrientation.portraitDown])
      .then((_) {
    runApp(MyApp());
  });
}

{ Note: The 'setPreferredOrientations' method returns a Future object. that's why you
shall wait until it's available and then move on with the application. Hence, 
there shall be used 'then' method }

Way 02: 
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For Android: 
Open android/app/src/main/AndroidManifest.xml and add
          android:screenOrientation="portrait"
            in your Main Activity
  
For iOS:
Open your project with xcode and restrict orientation for Potrait mode.