How to avoid "Some objects were not cleaned up when closing the scene. (Did you spawn new GameObjects from OnDestroy?)"

[KamilDev]

I think it's needed to unsubscribe from events in OnDestroy to prevent memory leaks, such as:

private void OnDestroy()
{
    if (StatsManager.Instance != null)
    {
        StatsManager.Instance.OnStatsChanged -= OnStatsChanged;
    }
}

This works, except for when exiting play mode, sometimes it will log:

Some objects were not cleaned up when closing the scene. (Did you spawn new GameObjects from OnDestroy?)

I believe it's because:

• Unity destroys StatsManager first.
• Unity next destroys the other GameObject that unsubcribes from StatsManager event.
• When checking StatsManager.Instance != null, it causes StatsManager to respawn since StatsManager.Instance is null. (Internal Singleton behavior)

What's the best way to avoid this issue?

[hasanbayatme]

I think a simple way to avoid this is to have a method to check whether there is any existing instance there without calling the get accessor on the Singleton class that initializes a new instance if there aren't any.

You could try it in your local.copy and see how it performs, like add a new method/property that just checks the internal instance field
of the singleton for null, and returns:

public bool HasInstance => instance != null;

This way, when you're unsubscribing for an event if the object is already destroyed, then no new object gets initialized by the singleton, as you're using the HasInstance in place of Instance in OnDestroy.

This should be only intended to be used in such cases as of OnDestroy or OnDisable where the instance it self could be destroyed due to Unity unloading the scene.