Unable to run hardcode-Tray in Debian testing

### Specifications
- Hardcode-Tray version : `4.3`
- Distribution : `Debian`
- Gtk version : `3.24`
- Inkscape version : `1.0`
- Cairo version : `1.16.0-4`
- Sni-qt version: `0.2.7+20161212`

### Steps to reproduce the problem

```
# → hardcode-tray
Unable to init server: Could not connect: Connection refused
Unable to init server: No se pudo conectar: Connection refused
Welcome to Hardcode-Tray!
Hardcode-Tray replaces some icons on the binary files of some applications.
Please close all the running applications before running Hardcode-Tray script.
Desktop Environment: Other
Scaling Factor: 1
Icon Size: 22
Traceback (most recent call last):
File "/usr/bin/hardcode-tray", line 103, in
if not isinstance(App.theme(), dict):
File "/usr/lib/python3.7/site-packages/HardcodeTray/app.py", line 177, in theme
theme = App.get("theme")
File "/usr/lib/python3.7/site-packages/HardcodeTray/app.py", line 77, in get
value = method()
File "/usr/lib/python3.7/site-packages/HardcodeTray/modules/config/system.py", line 67, in theme
theme_name = Gtk.Settings.get_default().get_property("gtk-icon-theme-name")
AttributeError: 'NoneType' object has no attribute 'get_property'
```

Reproduce the failure with the reported Debian, GTK, and Python versions, then start at /usr/bin/hardcode-tray line 103 and HardcodeTray/modules/config/system.py line 67. Trace why Gtk.Settings.get_default() is None; done means hardcode-tray starts without this traceback in the reported environment.

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