general: fix 'NoneType object has no attribute mode' error
Traceback (most recent call last):
File "/usr/lib/python3/dist-packages/gtweak/app.py", line 30, in do_activate
self.win = Window(self, model)
File "/usr/lib/python3/dist-packages/gtweak/tweakview.py", line 38, in __init__
self._model.load_tweaks(self)
File "/usr/lib/python3/dist-packages/gtweak/tweakmodel.py", line 104, in load_tweaks
mods = __import__("gtweak.tweaks", globals(), locals(), tweak_files, 0)
File "/usr/lib/python3/dist-packages/gtweak/tweaks/tweak_group_general.py", line 14, in <module>
_shell_not_ubuntu = _shell.mode != 'ubuntu'
AttributeError: 'NoneType' object has no attribute 'mode'
Edited by Alberts Muktupāvels