fixed issue

Author: Andrei Neagu

packages/service-library/src/servicelib/long_running_tasks/task.py

@@ -364,7 +364,10 @@ async def get_task_result(
         if tracked_task.result_field.error is not None:
             raise string_to_object(tracked_task.result_field.error)
 
-        return string_to_object(tracked_task.result_field.result)  # type: ignore[arg-type]
+        if tracked_task.result_field.result is None:
+            return None
+
+        return string_to_object(tracked_task.result_field.result)
 
     async def cancel_task(
         self, task_id: TaskId, with_task_context: TaskContext