adding a catch for no metrics selected

app.py

@@ -25,14 +25,19 @@ with st.sidebar.expander("Compare one or more metrics", expanded=True):
     metrics,
     default="rouge")
 
+
 loaded_metrics= []
 for metric in metric_names:
     metric =  load_metric(metric)
     loaded_metrics.append(metric)
 
+
 ### Single metric mode
+print(metric_names)
+if metric_names == []:
+    st.markdown("## Please choose one or more metrics.")
 
-if len(metric_names) == 1:
+elif len(metric_names) == 1:
     metric_name = metric_names[0]
     st.markdown("# You chose " + metric_name.upper())