minor bug fix

app.py

@@ -108,10 +108,10 @@ class DemoRunner:
 
 def setup(api_key, model_name, n_blocks, n_bowls):
     if not api_key:
-        return 'Please enter your OpenAI API key!', None
+        return 'Please enter your OpenAI API key!', None, None
     
     if n_blocks + n_bowls == 0:
-        return 'Please select at least one object!', None
+        return 'Please select at least one object!', None, None
 
     demo_runner = DemoRunner()