fix sizeOf usage

@@ -71,7 +71,7 @@ class _BarcodeScannerWithControllerState
       case AppLifecycleState.paused:
         return;
       case AppLifecycleState.resumed:
-        _subscription ??= controller.barcodes.listen(_handleBarcode);
+        _subscription = controller.barcodes.listen(_handleBarcode);
 
         unawaited(controller.start());
       case AppLifecycleState.inactive:

@@ -90,7 +90,7 @@ class _BarcodeScannerWithScanWindowState
   @override
   Widget build(BuildContext context) {
     final scanWindow = Rect.fromCenter(
-      center: MediaQuery.of(context).size.center(Offset.zero),
+      center: MediaQuery.sizeOf(context).center(Offset.zero),
       width: 200,
       height: 200,
     );

@@ -23,7 +23,7 @@ class _BarcodeScannerWithOverlayState extends State<BarcodeScannerWithOverlay> {
   @override
   Widget build(BuildContext context) {
     final scanWindow = Rect.fromCenter(
-      center: MediaQuery.of(context).size.center(Offset.zero),
+      center: MediaQuery.sizeOf(context).center(Offset.zero),
       width: 200,
       height: 200,
     );