Update app.py

app.py

@@ -115,7 +115,7 @@ def edit_mask_add(canvas,  image, idx, mask_np_list):
 
 def slider_release(index, image,  mask_np_list_updated, mask_label_list):
 
-    if index > len(mask_np_list_updated):
+    if index > len(mask_np_list_updated)-1:
         return image, "out of range"
     else:
         mask_np = mask_np_list_updated[index]