Default to main canvas in scene selection

New scene selections would have no canvas selected at all.
This would cause issues in places which do not support an empty canvas
selection.

For example, the "scene changed" check of the scene condition would not
work as without any canvas it is not clear which scene to check for.

lib/utils/canvas-helpers.cpp

@@ -297,6 +297,12 @@ int GetIndexOfScene(const OBSWeakCanvas &weakCanvas, const OBSWeakSource &scene)
 	return data.idx;
 }
 
+OBSWeakCanvas GetMainCanvas()
+{
+	OBSCanvasAutoRelease mainCanvas = obs_get_main_canvas();
+	return OBSGetWeakRef(mainCanvas);
+}
+
 #if LIBOBS_API_VER < MAKE_SEMANTIC_VERSION(31, 1, 0)
 
 obs_canvas_t *obs_canvas_get_ref(obs_canvas_t *)

lib/utils/canvas-helpers.hpp

@@ -42,6 +42,7 @@ EXPORT OBSWeakSource GetActiveCanvasScene(const OBSWeakCanvas &canvas);
 EXPORT OBSWeakSource GetSceneAtIndex(const OBSWeakCanvas &weakCanvas, int idx);
 EXPORT int GetIndexOfScene(const OBSWeakCanvas &weakCanvas,
 			   const OBSWeakSource &scene);
+EXPORT OBSWeakCanvas GetMainCanvas();
 
 class CanvasSelection : public FilterComboBox {
 	Q_OBJECT

lib/utils/scene-selection.cpp

@@ -103,10 +103,11 @@ void SceneSelection::Load(obs_data_t *obj, const char *name,
 	OBSDataAutoRelease data =
 		obs_data_get_obj(obj, selectionSaveName.data());
 
-	if (obs_data_has_user_value(data, canvasSaveName.data())) {
-		_canvas = GetWeakCanvasByName(
-			obs_data_get_string(data, canvasSaveName.data()));
-	} else {
+	const char *canvasName =
+		obs_data_get_string(data, canvasSaveName.data());
+	_canvas = GetWeakCanvasByName(
+		obs_data_get_string(data, canvasSaveName.data()));
+	if (strcmp(canvasName, "") == 0) {
 		OBSCanvasAutoRelease mainCanvas = obs_get_main_canvas();
 		_canvas = obs_canvas_get_weak_canvas(mainCanvas);
 		obs_weak_canvas_release(_canvas);

lib/utils/scene-selection.hpp

@@ -30,7 +30,7 @@ public:
 
 private:
 	OBSWeakSource _scene;
-	OBSWeakCanvas _canvas;
+	OBSWeakCanvas _canvas = GetMainCanvas();
 	SceneGroup *_group = nullptr;
 	std::weak_ptr<Variable> _variable;
 	Type _type = Type::SCENE;