Exception in stocklines.py

Mystery exception from the Dev:
2023-06-15 16:22:08,509 ERROR quicktill.till
  Exception caught at top level
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/quicktill/till.py", line 291, in run
    ui_gtk.run(
  File "/usr/lib/python3/dist-packages/quicktill/ui_gtk.py", line 627, in run
    tillconfig.mainloop.iterate()
  File "/usr/lib/python3/dist-packages/quicktill/event_glib.py", line 23, in iterate
    raise self._exc_info[0].with_traceback(
  File "/usr/lib/python3/dist-packages/quicktill/ui_gtk.py", line 102, in _keypress
    ui.handle_raw_keyboard_input(k)
  File "/usr/lib/python3/dist-packages/quicktill/ui.py", line 108, in handle_raw_keyboard_input
    handle_keyboard_input(k)
  File "/usr/lib/python3/dist-packages/quicktill/ui.py", line 86, in handle_keyboard_input
    basicwin._focus.hotkeypress(k)
  File "/usr/lib/python3/dist-packages/quicktill/ui.py", line 307, in hotkeypress
    self.parent.hotkeypress(k)
  File "/usr/lib/python3/dist-packages/quicktill/ui.py", line 307, in hotkeypress
    self.parent.hotkeypress(k)
  File "/usr/lib/python3/dist-packages/quicktill/ui.py", line 307, in hotkeypress
    self.parent.hotkeypress(k)
  [Previous line repeated 2 more times]
  File "/usr/lib/python3/dist-packages/quicktill/register.py", line 3290, in hotkeypress
    super().hotkeypress(k)
  File "/usr/lib/python3/dist-packages/quicktill/ui.py", line 405, in hotkeypress
    basicwin._focus.keypress(k)
  File "/usr/lib/python3/dist-packages/quicktill/ui.py", line 957, in keypress
    super().keypress(k)
  File "/usr/lib/python3/dist-packages/quicktill/ui.py", line 717, in keypress
    self.parent.keypress(k)
  File "/usr/lib/python3/dist-packages/quicktill/stocklines.py", line 119, in keypress
    line = self.s.dl.pop(self.s.cursor)
TypeError: 'NoneType' object cannot be interpreted as an integer
sde1000 / quicktill

Exception in stocklines.py #261
