The error now is happening on this line: self.notes_panel.items = [new_note]
If you look at what add_note_to_application returns, you’ll see it doesn’t return anything. So it’s returning None, which you’re adding as an element. When the data bindings try to index None to get field values you get the error.
Personally, since the number of notes looks like it’ll be relatively small on each application, I’d just return applicication_to_update[‘notes’] from add_note_to_application, and then on the client side just do self.notes_panel.items = new_notes.