Set ML worker to None after exiting

app.py

@@ -44,6 +44,7 @@ def run_ml_worker(url, api_key, hf_token):
     )
     args = ml_worker.args
     print(f"Process {args} exited with {ml_worker.wait()}")
+    ml_worker = None
 
 
 def stop_ml_worker():
@@ -51,6 +52,7 @@ def stop_ml_worker():
     if ml_worker is not None:
         print(f"Stopping ML worker for {previous_url}")
         ml_worker.terminate()
+        ml_worker = None
         print("ML worker stopped")
         return "ML worker stopped"
     return "ML worker not started"