Catch folder_name == None in onFolderSelected

[rodolakis]

Refreshing folder...

Traceback (most recent call last):
  File "/home/beams3/RODOLAKIS/src/mdaviz/mdaviz/mainwindow.py", line 214, in onRefresh
    self.onFolderSelected(current_folder)
  File "/home/beams3/RODOLAKIS/src/mdaviz/mdaviz/mainwindow.py", line 179, in onFolderSelected
    folder_path = Path(folder_name)
                  ^^^^^^^^^^^^^^^^^
  File "/home/beams/RODOLAKIS/.conda/envs/mdaviz/lib/python3.11/pathlib.py", line 871, in __new__
    self = cls._from_parts(args)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/home/beams/RODOLAKIS/.conda/envs/mdaviz/lib/python3.11/pathlib.py", line 509, in _from_parts
    drv, root, parts = self._parse_args(args)
                       ^^^^^^^^^^^^^^^^^^^^^^
  File "/home/beams/RODOLAKIS/.conda/envs/mdaviz/lib/python3.11/pathlib.py", line 493, in _parse_args
    a = os.fspath(a)
        ^^^^^^^^^^^^
TypeError: expected str, bytes or os.PathLike object, not NoneType
Aborted (core dumped)