Avoid NPE for flutter attach mDNS (#31926)

packages/flutter_tools/lib/src/commands/attach.dart

@@ -216,7 +216,9 @@ class AttachCommand extends FlutterCommand {
         }
       } else if ((device is IOSDevice) || (device is IOSSimulator)) {
         final MDnsObservatoryDiscoveryResult result = await MDnsObservatoryDiscovery().query(applicationId: appId);
-        observatoryUri = await _buildObservatoryUri(device, hostname, result.port, result.authCode);
+        if (result != null) {
+          observatoryUri = await _buildObservatoryUri(device, hostname, result.port, result.authCode);
+        }
       }
       // If MDNS discovery fails or we're not on iOS, fallback to ProtocolDiscovery.
       if (observatoryUri == null) {

packages/flutter_tools/test/commands/attach_test.dart

@@ -537,6 +537,17 @@ void main() {
       final int port = (await portDiscovery.query(applicationId: 'bar'))?.port;
       expect(port, 1234);
     });
+
+    testUsingContext('Query returns null', () async {
+      final MDnsClient client = getMockClient(
+        <PtrResourceRecord>[],
+         <String, List<SrvResourceRecord>>{},
+      );
+
+      final MDnsObservatoryDiscovery portDiscovery = MDnsObservatoryDiscovery(mdnsClient: client);
+      final int port = (await portDiscovery.query(applicationId: 'bar'))?.port;
+      expect(port, isNull);
+    });
   });
 }