error dialog if the barcode cannot be converted to JSON!

lib/barcode.dart

@@ -21,8 +21,16 @@ Future<void> scanQrCode(BuildContext context) async {
 
     print("Scanned: $result");
 
+    Map<String, dynamic> data;
+
     // Look for JSON data in the result...
-    final data = json.decode(result);
+    try {
+      data = json.decode(result);
+    } on FormatException {
+      showErrorDialog(context, "Bad barcode data", result);
+
+      return;
+    }
 
     showProgressDialog(context, "Querying Server", "Sending barcode data to server");
 

lib/widget/dialogs.dart

@@ -1,6 +1,25 @@
 
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/material.dart';
+import 'package:font_awesome_flutter/font_awesome_flutter.dart';
+
+void showErrorDialog(BuildContext context, String title, String description) {
+  showDialog(
+    context: context,
+    child: SimpleDialog(
+      title: ListTile(
+        title: Text("Error"),
+        leading: FaIcon(FontAwesomeIcons.exclamationCircle),
+      ),
+      children: <Widget>[
+        ListTile(
+          title: Text(title),
+          subtitle: Text(description)
+        )
+      ]
+    )
+  );
+}
 
 void showProgressDialog(BuildContext context, String title, String description) {
   showDialog(