Handle null result with qr code scanner (#1021)

Author: renefloor

dogfooding/lib/screens/home_screen.dart

@@ -418,7 +418,7 @@ class _JoinForm extends StatelessWidget {
         onPressed: () async {
           final result = await QrCodeScanner.scan(context);
 
-          if (context.mounted) {
+          if (context.mounted && result != null) {
             _handleJoinUrl(context, result);
           }
         },

dogfooding/lib/screens/qr_code_scanner.dart

@@ -13,12 +13,12 @@ class QrCodeScanner extends StatefulWidget {
   @override
   State<QrCodeScanner> createState() => _QrCodeScannerState();
 
-  static Future<String> scan(BuildContext context) async {
+  static Future<String?> scan(BuildContext context) async {
     final result = await Navigator.push(
       context,
       MaterialPageRoute<String>(builder: (_) => const QrCodeScanner()),
     );
-    return result!;
+    return result;
   }
 }