fixing error

app.py

@@ -35,6 +35,9 @@ def greet(topic):
         except wikipedia.PageError as e:
             continue
         break
+    if(len(topics) == 0):
+        return ["Please Type a Different Topic", gr.update(visible=True), gr.update(value="", visible=False)]
+
     print("Selected topic: ", topic)        
     print("Summary: ", summary)
     summary = summary.replace("\n", "")