phase3: util_web + shop_scroll (custom scroll) null-safe; global Key? param fix

lib/utils/double_back_to_close_app.dart

@@ -21,7 +21,7 @@ class DoubleBackToCloseApp extends StatefulWidget {
   /// Creates a widget that allows the user to close the app by double tapping
   /// the back-button.
   const DoubleBackToCloseApp({
-    Key key,
+    Key? key,
     @required this.snackBar,
     @required this.child,
   })  : assert(snackBar != null),