UI: Require selected source for Transform shortcut

Fixes #2468

UI/window-basic-main.cpp

@@ -6225,6 +6225,9 @@ void OBSBasic::on_actionEditTransform_triggered()
 	if (transformWindow)
 		transformWindow->close();
 
+	if (!GetCurrentSceneItem())
+		return;
+
 	transformWindow = new OBSBasicTransform(this);
 	transformWindow->show();
 	transformWindow->setAttribute(Qt::WA_DeleteOnClose, true);