Fix the error raised when ReactAgent is created without an explicit system message (#18041)

llama-index-core/llama_index/core/agent/workflow/react_agent.py

@@ -28,9 +28,9 @@ from llama_index.core.tools import AsyncBaseTool
 from llama_index.core.workflow import Context
 
 
-def default_formatter() -> ReActChatFormatter:
+def default_formatter(fields: dict) -> ReActChatFormatter:
     """Sets up a default formatter so that the proper react header is set."""
-    return ReActChatFormatter.from_defaults(context="some context")
+    return ReActChatFormatter.from_defaults(context=fields["system_prompt"])
 
 
 class ReActAgent(SingleAgentRunnerMixin, BaseWorkflowAgent):