add some null handling

@@ -146,7 +146,7 @@ class MobileScannerWebPlugin {
         'torchable': hasFlash
       };
     } catch (e) {
-      throw PlatformException(code: 'MobileScannerWeb', message: e.toString());
+      throw PlatformException(code: 'MobileScannerWeb', message: '$e');
     }
   }
 
@@ -166,7 +166,7 @@ class MobileScannerWebPlugin {
   Future<void> cancel() async {
     try {
       // Stop the camera stream
-      _localStream!.getTracks().forEach((track) {
+      _localStream?.getTracks().forEach((track) {
         if (track.readyState == 'live') {
           track.stop();
         }