Confusing in Multiplayer Networking uNet Client-Server

Hello there, I just start with UNET and i really confuse with some question listed below, please help me figure out. I would like to make Client-Server, it's mean no host, no same client server project, it should doing what it does, I separet each server, client to each project. - The problem was come when I think, like a MMO i joined before, Server only for transfer and sync message, game state, i don't know why i must have same scene as Client, like all tutorial, i don't want to use NetworkManager, NetworkManager HUD, so i start a new one. - I have client and server separate, so i don't need to care about isLocalClient right ? Every client now is remote client, so, there is a way to spawn Player on Server and another player can see it without server have same scene like client ? - Why my RPC geting NullReference when calling from server ? - There is any tutorial for client-server without default unity network manager and don't have "local client" on it ? - Am i wrong to follow this to make Online game ? - Currently, i would like to understand how to sync server-client, spawn object like i said above and following code. Error at void RpcChangeMap(string mapName) with NullReference ?! Here is the code :

ZMatchServer [Code] public class ZMatchServer : MonoBehavior {
public int listenPort; public int maxConnection; public bool enableNAT;

 public Text currentPlayer;
 public Text currentLog;
 public Text currentMsg;

 private int onlinePlayer;

 void Awake()
 {
     DontDestroyOnLoad(this);
 }

 void Start () 
 {
     NetworkServer.Listen(listenPort);
     NetworkServer.RegisterHandler(MsgType.Connect,OnClientConnected);
     NetworkServer.RegisterHandler(MsgType.Disconnect,OnClientDisconnected);

     NetworkServer.RegisterHandler(MessageType.Login,OnPlayerLogin);
     NetworkServer.RegisterHandler(MessageType.SelectCharacter,OnSelectCharacter);
 }

 void OnServerInitialized()
 {
     Debug.Log("Server Initialize completed! ");
     currentLog.text = "Server Initialize completed! ";
 }

 void OnPlayerLogin(NetworkMessage netMsg)
 {
     //Read username and password from client
     var loginMsg = netMsg.ReadMessage<LoginMessage>();
     string username = loginMsg.username;
     string password = loginMsg.password;

     //Send usm and pwd to check query in database
     ClientLoginCallback(username,password, netMsg.conn.connectionId);
     currentMsg.text = "Player send login " + username + " " + password;
 }

 void ClientLoginCallback(string username, string password, int connectionId)
 {
     if(username == "Bao" && password == "123")
     {
         LoginCallbackMessage loginClbMsg = new LoginCallbackMessage();
         loginClbMsg.authCode = LoginCallbackMessage.AuthCode.Sucessfull;
         NetworkServer.SendToClient(connectionId,MessageType.LoginAuthCode,loginClbMsg);
     }
     else if (username == "NotBao" && password == "321")
     {
         LoginCallbackMessage loginClbMsg = new LoginCallbackMessage();
         loginClbMsg.authCode = LoginCallbackMessage.AuthCode.Sucessfull;
         NetworkServer.SendToClient(connectionId,MessageType.LoginAuthCode,loginClbMsg);
     }
 }

 void OnSelectCharacter(NetworkMessage netMsg)
 {
     var selectCharMsg = netMsg.ReadMessage<SelectCharacterMessage>();
     string characterName = selectCharMsg.characterName;
     string characterId = selectCharMsg.characterID;
     RpcChangeMap("Shipping Ship");
     RpcSpawnCharacter(characterName,characterId);
 }

 [ClientRpc]
 void RpcSpawnCharacter(string charNam, string charID)
 {
     Vector3 position = new Vector3(0,10,0);
     Instantiate(ZGlobalAsset.Instance.Archer,position,ZGlobalAsset.Instance.Archer.transform.rotation);
 }

 [ClientRpc]
 void RpcChangeMap(string mapName)
 {
     Application.LoadLevelAsync(mapName);
 }
     
 void OnClientConnected(NetworkMessage netMsg)
 {
     onlinePlayer +=1;
     currentPlayer.text = onlinePlayer.ToString();
     currentLog.text = "Player connected with connection ID " + netMsg.conn.connectionId;
 }

 void OnClientDisconnected(NetworkMessage netMsg)
 {
     if(onlinePlayer > 0)
     {
         onlinePlayer -= 1;
         currentPlayer.text = onlinePlayer.ToString();
         currentLog.text = "Player disconnected";
     }
 }

}

ZMatchClient

public class ZMatchClient : MonoBehaviour { [SerializeField]private string serverURL; [SerializeField]private string serverIP; [SerializeField]private int serverPort;

 public bool isConnected;

 #region test login
 public string loginID;
 public string loginPwd;

 private NetworkIdentity mainCharNetId;
 #endregion

 NetworkClient m_Client;

 private static ZMatchClient instance;

 public static ZMatchClient Instance { get { return instance; } }

 void Awake()
 {
     instance = this;
     DontDestroyOnLoad(this);
 }

 void Start () 
 {
     ConnectToServer();
 }

 protected void ConnectToServer()
 {
     m_Client = new NetworkClient();
     m_Client.Connect(serverIP,serverPort);
     m_Client.RegisterHandler(MsgType.Connect, ClientConnect);
     m_Client.RegisterHandler(MsgType.Disconnect, ClientDisconnect);

     m_Client.RegisterHandler(MessageType.LoginAuthCode, LoginAuthCodeCallback);
 }

 //Login message
 protected void LoginToServer(string username, string password)
 {
     LoginMessage loginMsg = new LoginMessage();
     loginMsg.username = username;
     loginMsg.password = password;

     m_Client.Send(MessageType.Login,loginMsg);
 }

 private void LoginAuthCodeCallback(NetworkMessage netMsg)
 {
     var loginClbMsg = netMsg.ReadMessage<LoginCallbackMessage>();
     var authCode =    loginClbMsg.authCode;
     switch(authCode)
     {
     case LoginCallbackMessage.AuthCode.Sucessfull :
         Debug.Log("Login success");
         isConnected = true;
         SelectCharacter("Lancer","100");
         break;
     case LoginCallbackMessage.AuthCode.WrongUsername:
         Debug.Log("Wrong username");
         break;
     case LoginCallbackMessage.AuthCode.WrongPassword:
         Debug.Log("Wrong password");
         break;
     case LoginCallbackMessage.AuthCode.UnknowError :
         Debug.Log("Unknown error");
         break;
     }
 }

 #region Test client sync
 protected void SelectCharacter(string charName, string charID)
 {
     SelectCharacterMessage selectCharMsg = new SelectCharacterMessage();
     selectCharMsg.characterName = charName;
     selectCharMsg.characterID = charID;

     m_Client.Send(MessageType.SelectCharacter,selectCharMsg);
 }

 #endregion

 //Connected callback
 void ClientConnect(NetworkMessage netMsg)
 {
     Debug.Log("Client connected " + netMsg.msgType.ToString() + " " + netMsg.conn.connectionId);
     LoginToServer(loginID,loginPwd);
 }

 //Disconnected callback
 void ClientDisconnect(NetworkMessage netMsg)
 {
     Debug.Log("ClientDisconnect  " + netMsg.msgType.ToString() + " " + netMsg.conn.connectionId);

 }

}