Bug when changing sets

[Yokto13]

What?

Weird index error when changing between sets. Happens probably quite rarely when one set is much longer the the other.

Reproduce:

Use the sets provided in the repo by default. Create a new set with just one pack with one question. Go to set: site, pack: web_otazky.csv then change to the new set. It should crash with some tui related index error.

The result after crash:

Traceback (most recent call last):
  File "/home/arklez/codes/Git/sphinx/main_tui.py", line 233, in <module>
    res = main_loop()
  File "/home/arklez/codes/Git/sphinx/main_tui.py", line 70, in main_loop
    res = d.handle_input(key)
  File "/home/arklez/codes/Git/sphinx/venv/lib/python3.10/site-packages/picotui/basewidget.py", line 66, in handle_input
    res = self.handle_mouse(inp[0], inp[1])
  File "/home/arklez/codes/Git/sphinx/venv/lib/python3.10/site-packages/picotui/widgets.py", line 142, in handle_mouse
    return w.handle_mouse(x, y)
  File "/home/arklez/codes/Git/sphinx/venv/lib/python3.10/site-packages/picotui/widgets.py", line 322, in handle_mouse
    self.signal("changed")
  File "/home/arklez/codes/Git/sphinx/venv/lib/python3.10/site-packages/picotui/basewidget.py", line 28, in signal
    self.signals[sig](self)
  File "/home/arklez/codes/Git/sphinx/main_tui.py", line 165, in w_listbox_changed
    pack_listbox.redraw()
  File "/home/arklez/codes/Git/sphinx/venv/lib/python3.10/site-packages/picotui/editor.py", line 61, in redraw
    self.show_line(self.content[i], i)
IndexError: list index out of range