fix: disposed CameraController error thrown when changing a camera (flutter#4236)

Author: bselwe

packages/camera/camera/example/lib/main.dart

@@ -594,17 +594,21 @@ class _CameraExampleHomeState extends State<CameraExampleHome>
   }
 
   void onNewCameraSelected(CameraDescription cameraDescription) async {
-    if (controller != null) {
-      await controller!.dispose();
-    }
+    final previousCameraController = controller;
+
     final CameraController cameraController = CameraController(
       cameraDescription,
       ResolutionPreset.medium,
       enableAudio: enableAudio,
       imageFormatGroup: ImageFormatGroup.jpeg,
     );
+
     controller = cameraController;
 
+    if (mounted) {
+      setState(() {});
+    }
+
     // If the controller is updated then update the UI.
     cameraController.addListener(() {
       if (mounted) setState(() {});
@@ -637,6 +641,8 @@ class _CameraExampleHomeState extends State<CameraExampleHome>
     if (mounted) {
       setState(() {});
     }
+
+    await previousCameraController?.dispose();
   }
 
   void onTakePictureButtonPressed() {