Why does it return null when passing ScriptableObject through Inspector?

Question

Unity

[SerializeField] private LevelProps props;
ScriptableObject

[CreateAssetMenu(fileName = "levelProps", menuName = "levelProps", order = 0)]
    public class LevelProps : ScriptableObject
    {
        [SerializeField] public List<GameObject> planets;
        [SerializeField] public List<GameObject> asteroids;
        [SerializeField] public List<GameObject> massiveObjects;
        [SerializeField] public List<GameObject> massiveObjectsWholes;
        [SerializeField] public List<GameObject> stations;
    }

in the first code I throw through the scriptableObject inspector and there is null
the first code is in a separate file