App crashes

I use Debian 13 with Plasma desktop, Nvidia RTX, with driver version 550.163.01 and X11.

Launching the app gives me the following output:

```
~$ flatpak run io.github.jeffshee.Hidamari
INFO:Hidamari:Hidamari v3.6
--- System information ---
is_gnome = False
is_wayland = False
is_nvidia_proprietary = True
is_vdpau_ok = True
is_flatpak = True
--------------------------
INFO:Hidamari:[Args] {'p': 0, 'background': False, 'debug': False, 'reset': False}
Traceback (most recent call last):
File "/app/bin/hidamari", line 44, in
sys.exit(main.main(VERSION, pkgdatadir, localedir))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/app/share/hidamari/hidamari/__main__.py", line 56, in main
server.main(version, pkgdatadir, localedir, args)
File "/app/share/hidamari/hidamari/server.py", line 334, in main
server = HidamariServer(version, pkgdatadir, localedir, args)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/app/share/hidamari/hidamari/server.py", line 95, in __init__
self._load_config()
File "/app/share/hidamari/hidamari/server.py", line 107, in _load_config
self.config = ConfigUtil().load()
^^^^^^^^^^^^^^^^^^^
File "/app/share/hidamari/hidamari/utils.py", line 458, in load
self._checkDefaultSource(config)
File "/app/share/hidamari/hidamari/utils.py", line 433, in _checkDefaultSource
if mode == MODE_VIDEO and not os.path.isfile(default_source):
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "", line 30, in isfile
TypeError: stat: path should be string, bytes, os.PathLike or integer, not NoneType
```

Side note: It was the case with the previous Gnome platform too I just forgot to create a new issue related to this.

Start with hidamari/utils.py at ConfigUtil._checkDefaultSource and ConfigUtil.load, then follow the call from hidamari/server.py. Reproduce the Flatpak launch on the reported setup and determine why default_source is None; done means launching the app no longer raises the shown TypeError and the default-source behavior is covered by an appropriate test if one exists.

Written by the indexing model from the issue text.