I am getting the following message when I use roswtf command:

Traceback (most recent call last):
  File "/opt/ros/melodic/bin/roswtf", line 35, in <module>
    roswtf.roswtf_main()
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roswtf/__init__.py", line 89, in roswtf_main
    _roswtf_main()
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roswtf/__init__.py", line 138, in _roswtf_main
    static_plugins, online_plugins = roswtf.plugins.load_plugins()
  File "/opt/ros/melodic/lib/python2.7/dist-packages/roswtf/plugins.py", line 54, in load_plugins
    to_check = rospack.get_depends_on('roswtf', implicit=False)
  File "/usr/lib/python2.7/dist-packages/rospkg/rospack.py", line 268, in get_depends_on
    m = self.get_manifest(r)
  File "/usr/lib/python2.7/dist-packages/rospkg/rospack.py", line 167, in get_manifest
    return self._load_manifest(name)
  File "/usr/lib/python2.7/dist-packages/rospkg/rospack.py", line 211, in _load_manifest
    retval = self._manifests[name] = parse_manifest_file(self.get_path(name), self._manifest_name, rospack=self)
  File "/usr/lib/python2.7/dist-packages/rospkg/manifest.py", line 412, in parse_manifest_file
    _static_rosdep_view = init_rospack_interface()
  File "/usr/lib/python2.7/dist-packages/rosdep2/rospack.py", line 59, in init_rospack_interface
    lookup = _get_default_RosdepLookup(Options())
  File "/usr/lib/python2.7/dist-packages/rosdep2/main.py", line 130, in _get_default_RosdepLookup
    verbose=options.verbose)
  File "/usr/lib/python2.7/dist-packages/rosdep2/sources_list.py", line 633, in create_default
    sources = load_cached_sources_list(sources_cache_dir=sources_cache_dir, verbose=verbose)
  File "/usr/lib/python2.7/dist-packages/rosdep2/sources_list.py", line 533, in load_cached_sources_list
    return parse_sources_data(cache_data, origin=cache_index, model=model)
  File "/usr/lib/python2.7/dist-packages/rosdep2/sources_list.py", line 388, in parse_sources_data
    raise InvalidData('line:\n\t%s\n%s' % (line, e), origin=origin)
rosdep2.core.InvalidData: line:
    yaml https://raw.githubusercontent.com/ros/rosdistro/master/rosdep/osx-homebrew.yaml osx
unsupported pickle protocol: 4

I am using ros-melodic in Ubuntu 18.04.