Fixed screen going black after hot reload.

example/lib/main.dart

@@ -24,6 +24,12 @@ class _QRViewExampleState extends State<QRViewExample> {
   QRViewController controller;
   final GlobalKey qrKey = GlobalKey(debugLabel: 'QR');
 
+  @override
+  void reassemble() {
+    super.reassemble();
+    controller.pauseCamera();
+  }
+
   @override
   Widget build(BuildContext context) {
     return Scaffold(