Fix: Prevent UnboundLocalError in on_populate_tree_lists

- Added a safeguard to skip items in `user_lists` that do not match expected types (`UserPlaylist`, `Playlist`, or `Mix`).
- Ensured `twi_child` is only accessed when properly initialized.
- This prevents crashes during GUI interaction when unexpected data is encountered.

tidal_dl_ng/gui.py

@@ -360,6 +360,8 @@ class MainWindow(QtWidgets.QMainWindow, Ui_MainWindow):
                 twi_child = QtWidgets.QTreeWidgetItem(twi_mixes)
                 name: str = item.title
                 info: str = item.sub_title
+            else:
+                continue
 
             twi_child.setText(0, name)
             set_user_list_media(twi_child, item)