Add verification of number of items in the table to prevent editing an empty object with no index

src/main.py

@@ -123,6 +123,7 @@ class MedTrackerApp:
 
     def on_double_click(self, event):
         """Handle double-click event to edit an entry."""
+        if len(self.tree.get_children()) > 0:
             item_id = self.tree.selection()[0]
             item_values = self.tree.item(item_id, "values")
             self.create_edit_window(item_id, item_values)