Getting Things Gnome synchronisation methods can't be added

[necessarily-equal]

Describe the bug

The synchronisation methods in getting things done (the gtg package) are not usable because they can't be added.

Steps To Reproduce

Steps to reproduce the behavior:

1. start the app
2. click on the hamburger menu
3. cilck on "Synchronisation" in the menu
4. click on the [+] button on the bottom left

**Context:** Global generic exception

\```python-traceback
Traceback (most recent call last):
  File "/nix/store/4vbaaxbsman57px5pcv28pzwjnns2gk6-gtg-0.6/lib/python3.11/site-packages/GTG/gtk/backends/__init__.py", line 231, in on_add_button
    self.add_panel.refresh_backends()
  File "/nix/store/4vbaaxbsman57px5pcv28pzwjnns2gk6-gtg-0.6/lib/python3.11/site-packages/GTG/gtk/backends/addpanel.py", line 141, in refresh_backends
    self.combo_types.refresh()
  File "/nix/store/4vbaaxbsman57px5pcv28pzwjnns2gk6-gtg-0.6/lib/python3.11/site-packages/GTG/gtk/backends/backendscombo.py", line 81, in refresh
    pixbuf = self.dialog.get_pixbuf_from_icon_name(module.Backend.get_icon(), 16)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/4vbaaxbsman57px5pcv28pzwjnns2gk6-gtg-0.6/lib/python3.11/site-packages/GTG/gtk/backends/__init__.py", line 121, in get_pixbuf_from_icon_name
    return Gtk.IconTheme.get_default().load_icon(name, height, 0)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
gi.repository.GLib.GError: gtk-icon-theme-error-quark: Icon 'applications-internet' not present in theme Adwaita (0)
\```

**Software versions:**
* Getting Things GNOME! 0.6.0
* CPython 3.11.9 (main, Apr  2 2024, 08:25:04) [GCC 13.2.0]
* GTK 3.24.41, GLib 2.78.4
* PyGLib 3.46.0, PyGObject 3.46.0
* Linux-6.8.6-zen1-x86_64-with-glibc2.39

I tested both with NixOS 23.11 and Unstable

Expected behavior

A clear and concise description of what you expected to happen.

Screenshots

Notify maintainers

@oyren

[necessarily-equal]

This seems to be a purely graphical problem.

One thing I tried to fix it, inspired by https://stackoverflow.com/questions/35794241/meld-gi-glib-gerror-icon-meld-change-apply-right-not-present-in-theme-wha#53890252, is to put gnome-icon-theme in propagatedBuildInputs but that didn't work.

[necessarily-equal]

It seems to be a known issue upstream. There seem to be an issue opened, but the url 404 at least for me:

• Find and integrate a substitute for the deprecated "applications-internet" icon used by the CalDAV sync backend (causes an Internal error with Adwaita 42) getting-things-gnome/gtg#846

Closed as duplicates:

• Issues when trying to add a synchronization service getting-things-gnome/gtg#947
• Internal error when adding syncronisation getting-things-gnome/gtg#934
• Internal error when trying to add a new sync service getting-things-gnome/gtg#896
• Flatpak: cant enable synchronization getting-things-gnome/gtg#879

I just asked there getting-things-gnome/gtg#934 (comment) for more info.