correct callback

app.py

@@ -143,7 +143,7 @@ def stream(input_text) -> Generator:
         try:
             next_token = q.get(True, timeout=60)
             print("next_token: ", next_token)
-            if len(next_token) ==0:
+            if next_token == None:
                 next_token = ""
             # counter = counter + 1
             # print("No data, retry number: ", counter)