How to cast SerializedProperty back to custom class?

I have a list of modelAsset in type of SerializedProperty and I want to cast them back to modelAsset.

 ModelAsset.cs
 ----------------------
 [Serializable]
 public struct modelAsset {
     public string modelName;
     public GameObject Prefab;
 
     public List<GameObject> up;
     public List<GameObject> down;
     public List<GameObject> back;
     public List<GameObject> forward;
     public List<GameObject> right;
     public List<GameObject> left;
 }

I want to take the first item in the list and cast it back to its original class but unity says The as operator must be used with a reference type or nullable type (''modelAsset" is a non-nullable type) to me

         modelAsset modelAss = list.serializedProperty.GetArrayElementAtIndex(0).objectReferenceValue as System.Object as modelAsset;

What I currently can do is:

         listItem.FindPropertyRelative("up").GetArrayElementAtIndex(0).objectReferenceValue as GameObject);

Which can return the first GameObject from its "up" named List.