## Versions
- OS: Linux
- GTK+ Version: 3.24.7
- PyGObject Version: 3.28.3
- python-nbxmpp Version: 0.6.10
- Gajim Version: 1.1.3

## Traceback
```
Traceback (most recent call last):
 File "/gnu/store/lzzphfg2hf52h1cb2dbvz3qvz2ca26na-gajim-1.1.3/lib/python3.7/site-packages/gajim/application.py", line 221, in _activate
 self.interface.run(self)
 File "/gnu/store/lzzphfg2hf52h1cb2dbvz3qvz2ca26na-gajim-1.1.3/lib/python3.7/site-packages/gajim/gui_interface.py", line 2550, in run
 app.plugin_manager = plugins.PluginManager()
 File "/gnu/store/lzzphfg2hf52h1cb2dbvz3qvz2ca26na-gajim-1.1.3/lib/python3.7/site-packages/gajim/plugins/helpers.py", line 129, in __call__
 cls.instance = super(Singleton, cls).__call__(*args, **kwargs)
 File "/gnu/store/lzzphfg2hf52h1cb2dbvz3qvz2ca26na-gajim-1.1.3/lib/python3.7/site-packages/gajim/plugins/pluginmanager.py", line 115, in __init__
 pc = self.scan_dir_for_plugins(path)
 File "/gnu/store/lzzphfg2hf52h1cb2dbvz3qvz2ca26na-gajim-1.1.3/lib/python3.7/site-packages/gajim/plugins/helpers.py", line 114, in wrapper
 result = f(*args, **kwargs)
 File "/gnu/store/lzzphfg2hf52h1cb2dbvz3qvz2ca26na-gajim-1.1.3/lib/python3.7/site-packages/gajim/plugins/pluginmanager.py", line 598, in scan_dir_for_plugins
 if not os.path.isdir(path):
 File "/gnu/store/h8l1pby3cm6b4fxsfwwr65b4d1hyh6cs-python-3.7.0/lib/python3.7/genericpath.py", line 42, in isdir
 st = os.stat(s)
TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType

```
## Steps to reproduce the problem

Install and open the app. This bug error shows up.