Always initialize source_model in _populate_source_model

When the OBS connection is unavailable, `_populate_source_model`
returns early without setting `self.source_model`.  This causes an
`AttributeError` when `load_config_defaults` subsequently calls
`self.source_model.get_n_items()`, breaking the configuration UI.

This patch moves the `Gio.ListStore()` initialization to the top of
`_populate_source_model`, before the connection check and early
return.  After this change, `load_config_defaults` sees an empty
model (zero items) and safely falls through to the `else` branch
without error.

Author: pniedzielski

actions/TimerSourceActionCore.py

@@ -88,12 +88,12 @@ def on_change_source(self, sources, *args):
             self.set_settings(settings)
 
     def _populate_source_model(self):
+        self.source_model = Gio.ListStore()
+
         if not self.plugin_base.get_connected():
             self.reconnect_obs()
         if not self.plugin_base.get_connected():
             return
-
-        self.source_model = Gio.ListStore()
         sources = sorted(
             self.plugin_base.backend.get_all_livesplit_one_sources(),
             key=lambda source: source["name"]