UI: Ignore scene source which has been removed

While getting source list, we should ignore scene source which has been
removed. If we don't ignore it, a deleted scene source will be added
later. Besides, signals of that scene have been sent before, such as
"remove" of scene. So we can't remove this scene source by "auto
removeItem" defined in SourceTreeItem::ReconnectSignals().

Fixes obsproject/obs-studio#2602

UI/source-tree.cpp

@@ -608,6 +608,11 @@ static bool enumItem(obs_scene_t *, obs_sceneitem_t *item, void *ptr)
 	QVector<OBSSceneItem> &items =
 		*reinterpret_cast<QVector<OBSSceneItem> *>(ptr);
 
+	obs_source_t *src = obs_sceneitem_get_source(item);
+	if (obs_source_removed(src)) {
+		return true;
+	}
+
 	if (obs_sceneitem_is_group(item)) {
 		obs_data_t *data = obs_sceneitem_get_private_settings(item);