Fix port error

app.py

@@ -358,4 +358,7 @@ with demo:
 
   demo.load(load_chain, [api_key, api_type], [chain, llm, chain_intro, chain_body1])
 
-demo.launch(debug=True, server_port=int(os.environ.get("SERVER_PORT", None)))
+port = os.environ.get("SERVER_PORT", None)
+if port:
+  port = int(port)
+demo.launch(debug=True, server_port=port)