Use scriptable object to refernce object. This doesn't require the object to have spawned, you can just listen for the OnValueChange (and check if value is already set)
Reference
[CreateAssetMenu(fileName = "NetworkIdentityReference", menuName = "Component Reference/NetworkIdentity")]
public class NetworkIdentityReference : ScriptableObject
{
[SerializeField] NetworkIdentity _value;
public event Action<NetworkIdentity> OnValueChange;