modify app to handle NoneType error

app.py

@@ -499,7 +499,14 @@ def call_agent(message: str, session_id: str, max_retries: int = 3) -> tuple[str
                 {"messages": [{"role": "user", "content": message}]},
                 config=config,
             )
-            for msg in reversed(result.get("messages", [])):
+            if not result:
+                return "Agent returned empty result. Please try again.", current_sid
+            
+            messages = result.get("messages", [])
+            if messages is None:
+                messages = []
+                
+            for msg in reversed(messages):
                 if hasattr(msg, "type") and msg.type == "ai":
                     return safe_str(msg.content), current_sid
                 if isinstance(msg, dict) and msg.get("role") in ("assistant", "ai"):
@@ -508,6 +515,7 @@ def call_agent(message: str, session_id: str, max_retries: int = 3) -> tuple[str
 
         except Exception as e:
             err = str(e)
+            tb = traceback.format_exc()
 
             # ── FIX-A: Corrupted history (dangling tool call in MemorySaver) ──
             # Rotate to a new thread so MemorySaver starts fresh.
@@ -530,15 +538,24 @@ def call_agent(message: str, session_id: str, max_retries: int = 3) -> tuple[str
                         {"messages": [{"role": "user", "content": recovery_msg}]},
                         config=config,
                     )
-                    for msg in reversed(result.get("messages", [])):
+                    
+                    if not result:
+                        return "Agent returned empty result after rotation.", current_sid
+                    
+                    messages = result.get("messages", [])
+                    if messages is None:
+                        messages = []
+                        
+                    for msg in reversed(messages):
                         if hasattr(msg, "type") and msg.type == "ai":
                             return safe_str(msg.content), current_sid
                         if isinstance(msg, dict) and msg.get("role") in ("assistant", "ai"):
                             return safe_str(msg.get("content", "")), current_sid
                     return "Agent returned no response after history rotation. Please try again.", current_sid
                 except Exception as e2:
+                    tb2 = traceback.format_exc()
                     log_error(str(e2), ctx="call_agent [post-rotation]")
-                    return f"⚠️ Agent Error after session rotation: {e2}\n\nSee error.txt for details.", current_sid
+                    return f"⚠️ Agent Error after session rotation: {e2}\n\nTraceback:\n{tb2}", current_sid
 
             # ── FIX-B: Mistral rate-limit / server errors — extended back-off ──
             if any(c in err for c in ["429", "520", "502", "503", "529", "mistral.ai", "Rate limit"]):
@@ -549,7 +566,7 @@ def call_agent(message: str, session_id: str, max_retries: int = 3) -> tuple[str
                 continue
 
             log_error(err, ctx="call_agent")
-            return f"⚠️ Agent Error: {err}\n\nSee error.txt for details.", current_sid
+            return f"⚠️ Agent Error: {err}\n\nTraceback:\n{tb}", current_sid
 
     return "❌ Mistral not responding after retries. Wait a few minutes and try again.", current_sid