Invalid use of prepare-upload crashes git-ubuntu (Traceback)

Using git-ubuntu 1.1-20-g2c19860 (edge), I noticed that calling

$ git ubuntu prepare-upload

leads to a traceback. That usage is clearly invalid, but git-ubuntu should give a useful help message instead of crashing (compare with what e.g. `git clone` does). The full traceback follows.

--

Traceback (most recent call last):
  File "/snap/git-ubuntu/1302/usr/bin/git-ubuntu", line 11, in <module>
    load_entry_point('gitubuntu==1.1', 'console_scripts', 'git-ubuntu')()
  File "/snap/git-ubuntu/1302/usr/lib/python3/dist-packages/gitubuntu/__main__.py", line 191, in main
    args = parser.parse_args()
  File "/snap/git-ubuntu/1302/usr/lib/python3.8/argparse.py", line 1780, in parse_args
    args, argv = self.parse_known_args(args, namespace)
  File "/snap/git-ubuntu/1302/usr/lib/python3.8/argparse.py", line 1812, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/snap/git-ubuntu/1302/usr/lib/python3.8/argparse.py", line 2021, in _parse_known_args
    stop_index = consume_positionals(start_index)
  File "/snap/git-ubuntu/1302/usr/lib/python3.8/argparse.py", line 1977, in consume_positionals
    take_action(action, args)
  File "/snap/git-ubuntu/1302/usr/lib/python3.8/argparse.py", line 1886, in take_action
    action(self, namespace, argument_values, option_string)
  File "/snap/git-ubuntu/1302/usr/lib/python3.8/argparse.py", line 1171, in __call__
    subnamespace, arg_strings = parser.parse_known_args(arg_strings, None)
  File "/snap/git-ubuntu/1302/usr/lib/python3.8/argparse.py", line 1812, in parse_known_args
    namespace, args = self._parse_known_args(args, namespace)
  File "/snap/git-ubuntu/1302/usr/lib/python3.8/argparse.py", line 2047, in _parse_known_args
    ', '.join(required_actions))
TypeError: sequence item 0: expected str instance, NoneType found