Tiny fix for `ExecutionManager.execution_from_child_message`

shared/workspaces/workspaces/capability/services/execution_manager.py

@@ -201,8 +201,9 @@ class ExecutionManager(ExecutionManagerIF):
 
         if msg_subject.get("execution_wf_id"):
             parent_wf_request_id = msg_subject.get("execution_wf_id")
-        elif message["argument"].get("parent_wf_request_id"):
-            parent_wf_request_id = message["argument"].get("parent_wf_request_id")
+        elif wf_arguments := message.get("argument"):
+            if wf_arguments.get("parent_wf_request_id"):
+                parent_wf_request_id = message["argument"].get("parent_wf_request_id")
 
         if parent_wf_request_id:
             # Workflow request has a parent_wf_request_id; use that as the request ID instead of the true ID