no off-target bug

app.py

@@ -26,8 +26,11 @@ elif all([True if nt.upper() in NUCLEOTIDE_TOKENS.keys() else False for nt in st
     on_target, off_target = tiger_exhibit(st.session_state['userInput'])
     st.write('On-target predictions: ', on_target)
     st.download_button(label='Download', data=convert_df(on_target), file_name='on_target.csv', mime='text/csv')
-    st.write('Off-target predictions: ', off_target)
-    st.download_button(label='Download', data=convert_df(off_target), file_name='off_target.csv', mime='text/csv')
+    if len(off_target) > 0:
+        st.write('Off-target predictions: ', off_target)
+        st.download_button(label='Download', data=convert_df(off_target), file_name='off_target.csv', mime='text/csv')
+    else:
+        st.write('We did not find any off-target effects!')
 
 # invalid input
 else:

tiger.py

@@ -128,6 +128,8 @@ def find_off_targets(guides, batch_size=1000):
 
 
 def predict_off_target(off_targets: pd.DataFrame, model: tf.keras.Model):
+    if len(off_targets) == 0:
+        return pd.DataFrame()
 
     # append predictions off-target predictions
     model_inputs = tf.concat([