Reference to NetworkBehaviour that is not currently spawned on client (useful for synclists)

PlayerReference.cs

// player script  (in its own file)
public class Player : NetworkBehaviour { }

// reference to Player that can be sent over network
public struct PlayerReference
{
    public uint netid;
    private Player _player;

    public PlayerReference(Player value) : this()
    {
        _player = value;
        if (value != null)
            netid = value.netId;
    }

    public Player Player
    {
        get
        {
            if (_player == null)
            {
                NetworkIdentity identity = NetworkIdentity.spawned[netid];
                if (identity != null)
                {
                    _player = identity.GetComponent<Player>();
                }
            }
            return _player;
        }
    }

    public static implicit operator Player(PlayerReference reference) => reference.Player;
    public static implicit operator PlayerReference(Player value) => new PlayerReference(value);
}