let web throw when start is called once the barcode reader is set

@@ -280,6 +280,9 @@ class MobileScannerController extends ValueNotifier<MobileScannerState> {
         );
       }
     } on MobileScannerException catch (error) {
+      // TODO: if the error code is `controllerAlreadyInitialized` ignore the error
+      // TODO: update the error reporting from the native side to report proper error codes.
+
       // The initialization finished with an error.
       // To avoid stale values, reset the output size,
       // torch state and zoom scale to the defaults.

@@ -253,6 +253,17 @@ class MobileScannerWeb extends MobileScannerPlatform {
 
   @override
   Future<MobileScannerViewAttributes> start(StartOptions startOptions) async {
+    // TODO: ignore double starts in the controller.
+    if (_barcodeReader != null) {
+      throw const MobileScannerException(
+        errorCode: MobileScannerErrorCode.controllerAlreadyInitialized,
+        errorDetails: MobileScannerErrorDetails(
+          message:
+              'The scanner was already started. Call stop() before calling start() again.',
+        ),
+      );
+    }
+
     _barcodeReader = ZXingBarcodeReader();
 
     await _barcodeReader?.maybeLoadLibrary(
@@ -260,6 +271,7 @@ class MobileScannerWeb extends MobileScannerPlatform {
     );
 
     if (_barcodeReader?.isScanning ?? false) {
+      // TODO: ignore double starts in the controller.
       throw const MobileScannerException(
         errorCode: MobileScannerErrorCode.controllerAlreadyInitialized,
         errorDetails: MobileScannerErrorDetails(