Empty case list handling causes curitz traceback

[runborg]

When you have a completely empty case-list, eg. after filtering with a random nonexistant string.
if you then press "[ENTER]", this tracebacks curitz with the error:

Traceback (most recent call last):
  File "/home/runarb/.local/bin/curitz", line 8, in <module>
    sys.exit(main())
  File "/home/runarb/.local/pipx/venvs/curitz/lib/python3.10/site-packages/curitz/cli.py", line 1182, in main
    curses.wrapper(uiloop, config)
  File "/usr/lib/python3.10/curses/__init__.py", line 94, in wrapper
    return func(stdscr, *args, **kwds)
  File "/home/runarb/.local/pipx/venvs/curitz/lib/python3.10/site-packages/curitz/cli.py", line 404, in uiloop
    runner(screen, config)
  File "/home/runarb/.local/pipx/venvs/curitz/lib/python3.10/site-packages/curitz/cli.py", line 845, in runner
    uiShowHistory(screen, lb.active.id, config)
  File "/home/runarb/.local/pipx/venvs/curitz/lib/python3.10/site-packages/curitz/culistbox.py", line 118, in active
    return self.elements[self.active_element]
IndexError: list index out of range

This should instead just do nothing.