A scene that has no groups will quietly fail when run by a ReactorSensor (it's logged, but there's no indication in the UI). The scene is therefore left in "unfinished" state in the scene state data. Later, when Luup restarts, Reactor sees the unfinished scene during its startup work, and attempts to run the scene groups, causing a startup crash (Reactor is attempting to iterate over an empty/none-existent array).
A scene that has no groups will quietly fail when run by a ReactorSensor (it's logged, but there's no indication in the UI). The scene is therefore left in "unfinished" state in the scene state data. Later, when Luup restarts, Reactor sees the unfinished scene during its startup work, and attempts to run the scene groups, causing a startup crash (Reactor is attempting to iterate over an empty/none-existent array).