Wrap command crashes the TUI when viewport is empty

When the viewport is empty, triggering the Wrap command crashes the TUI with a barely useful error traceback:

Traceback (most recent call last):
  File "/usr/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/home/max/Git/cobib/cobib/__main__.py", line 74, in <module>
    main()
  File "/home/max/Git/cobib/cobib/__main__.py", line 49, in main
    tui()
  File "/home/max/Git/cobib/cobib/tui/__init__.py", line 14, in tui
    curses.wrapper(TUI)
  File "/usr/lib/python3.8/curses/__init__.py", line 105, in wrapper
    return func(stdscr, *args, **kwds)
  File "/home/max/Git/cobib/cobib/tui/tui.py", line 185, in __init__
    self.loop()
  File "/home/max/Git/cobib/cobib/tui/tui.py", line 396, in loop
    self.viewport.chgat(self.current_line, 0,
_curses.error: chgat() returned ERR