chore: Update app.py and remove unnecessary code

app.py

@@ -34,13 +34,6 @@ API_URL = os.environ.get("API_URL")
 TOKEN = os.environ.get("TOKEN")
 TOKENIZER_REPO = "MediaTek-Research/Breeze-7B-Instruct-v1_0"
 MODEL_NAME = os.environ.get("MODEL_NAME")
-
-HEADERS = {
-    "Authorization": f"Bearer {TOKEN}", 
-    "Content-Type": "application/json",
-    "accept": "application/json"
-}
-
 MAX_SEC = 30
 MAX_INPUT_LENGTH = 5000
 
@@ -174,6 +167,8 @@ with gr.Blocks() as demo:
                 for delta in r:
                     if history[-1][1] is None:
                         history[-1][1] = ''
+                    if delta is None:
+                        delta = ''
                     history[-1][1] += delta
                     yield history
 
@@ -203,6 +198,7 @@ with gr.Blocks() as demo:
         ],
         outputs=chatbot
     )
+    
     submit_button.click(
         user, [msg, chatbot], [msg, chatbot], queue=False
     ).then(