nullreferenceexception error in my script

[Kinjiro26]

Hi guys im kinda new with unity. Im getting an null reference exception in my code. Here is my code:

public class LoginUI : InputAwareWindow
{

[Inject]
Private AccountManager _xAccountManager;
    void Start ()
    {
      Username.text = _xaccount_manager.GetLastUsername();
      Password.text = _xaccount_manager.GetLastPassword();	
	
	if ( string.IsNullOrEmpty(Error.text) && string.IsNullOrEmpty(Username.text) )
	{
		Error.text = HelpText;
	}
    }

     void OnLogin()
     {
           if ( !string.IsNullOrEmpty(Username.text) )		
		   {
			
			_xaccount_manager.Login(Username.text, Password.text); <- error here
 		
    		NGUITools.SetActive(MyWaitUI, true);
		}
		else
		{
			Error.text = HelpText;
		}
      }
   }

Public class AccountManager : Basebehavior
{
    public void Login(string username, string password)
	{
		_username = username;
		_password = password;
		
		Dictionary<string, string> data = new Dictionary<string, string>();
		data["username"] = username;
		data["password"] = password;
		
		_server_link.Request("login", OnLoginResult, data);
	}
     
 }

im getting the error in line 19 the error is :

NullReferenceException: Object reference not set to an instance of an object
LoginUI.Start () (at Assets/VSB/Scripts/LoginUI.cs:228)

Thank you. Your help is much appreciated

[salex100m]

try making your Start() into Awake() instead if you are using it just to initialize.