fix: add null safety checks in webhook response status hook (#12114)

### What problem does this PR solve?

Add optional chaining operators to prevent runtime errors when formData
is undefined or null in useShowWebhookResponseStatus hook.

This fixes a potential crash when accessing mode and execution_mode
properties before formData is initialized or when the Begin node doesn't
exist in the graph.

🤖 Generated with [Claude Code](https://claude.com/claude-code)


### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

Co-authored-by: Claude <noreply@anthropic.com>

web/src/pages/agent/form/message-form/use-show-response-status.ts

@@ -16,7 +16,7 @@ export function useShowWebhookResponseStatus(form: UseFormReturn<any>) {
     const formData: BeginFormSchemaType = getNode(BeginId)?.data.form;
     return (
       formData?.mode === AgentDialogueMode.Webhook &&
-      formData.execution_mode === WebhookExecutionMode.Streaming
+      formData?.execution_mode === WebhookExecutionMode.Streaming
     );
   }, [getNode]);