Custom NetworkManager script

Questions & Answers
1

I have Two Players network game and also a have a Custom Network Manager script for this.
Menu for this games looks like this
70155-screen-shot-2016-05-16-at-75008-am.png

CustomNetworkManager script:

using UnityEngine;
using System.Collections;
using UnityEngine.Networking;
using UnityEngine.UI;

public class NetworkManager_Custom : NetworkManager {

	[SerializeField] Vector3 playerSpawnPos;
	[SerializeField] GameObject Helicopter;
	[SerializeField] GameObject Soldier;

	bool soldier = true,helicop = true;

	GameObject choosenCharacter; // Helicopter or Soldier


	public void HelicopterChoose()
	{
		if (helicop) {
			choosenCharacter = Helicopter;
			playerSpawnPos = new Vector3 (-112.7f, 4f, -121.4f);
			helicop = false;
			JoinGame ();
		}
	}

	public void SoldierChoose()
	{
		if (soldier) {
			choosenCharacter = Soldier;
			playerSpawnPos = new Vector3 (0f, 0f, 0f);
			soldier = false;
			JoinGame ();
		}
	}
		
	public void StartupHost()
	{
		SetPort();
		NetworkManager.singleton.StartHost ();
	}

	public void JoinGame()
	{
		SetIPAddress ();
		SetPort ();
		NetworkManager.singleton.StartClient ();
	}
		
	void SetIPAddress()
	{
		string ipAdress = GameObject.Find ("InputFieldIPAddress").transform.FindChild ("Text").GetComponent<Text> ().text;
		NetworkManager.singleton.networkAddress = ipAdress;

	}

	void SetPort()
	{
		NetworkManager.singleton.networkPort = 7777;
	}

	void OnLevelWasLoaded(int level)
	{
		if (level == 0) {

			SetupMenuSceneButton ();
		}
	}
		
	void SetupMenuSceneButton()

	{
		GameObject.Find ("ButtonStartHost").GetComponent<Button> ().onClick.RemoveAllListeners ();
		GameObject.Find ("ButtonStartHost").GetComponent<Button> ().onClick.AddListener (StartupHost);

		GameObject.Find ("ButtonHelicoppter").GetComponent<Button> ().onClick.RemoveAllListeners ();
		GameObject.Find ("ButtonHelicoppter").GetComponent<Button> ().onClick.AddListener (HelicopterChoose);

		GameObject.Find ("ButtonSoldier").GetComponent<Button> ().onClick.RemoveAllListeners ();
		GameObject.Find ("ButtonSoldier").GetComponent<Button> ().onClick.AddListener (SoldierChoose);

	}
		
	public override void OnServerAddPlayer(NetworkConnection conn, short playerControllerId) {

		var player = (GameObject)GameObject.Instantiate(choosenCharacter, playerSpawnPos, Quaternion.identity);

		NetworkServer.AddPlayerForConnection(conn, player, playerControllerId);
	}
}

When i run and push Start Host i have an error:
ArgumentException: The thing you want to instantiate is null.
UnityEngine.Object.CheckNullArgument (System.Object arg, System.String message)

2

You should drag the player prefab to the slot of your custom networkmanager panel.