Object reference not set to an instance of an object

Hello Unity Community i came over this problem when i set up my mana script, it seems to get the error everytime i reach next Level, cause it raises "the damage" and "MaxMana", anyways heres the error:

NullReferenceException: Object reference not set to an instance of an object Boo.Lang.Runtime.RuntimeServices.InvokeBinaryOperator (System.String operatorName, System.Object lhs, System.Object rhs) KillSys.UpDamage () (at Assets/Scripts/KillSys.js:39) KillSys.KillCounter () (at Assets/Scripts/KillSys.js:25) Gui track taret.Dead () (at Assets/Scripts/Gui track taret.js:14)

The "KillSys" counts the kills, XP and Levels and displays it on screen and the "Gui track taret" is the script for the enemy's healthbar,

And Heres the KillSys script cause that seems to make the problem:

 static var PlayerXP : int;
 static var XPText = "XP: 0";
 static var MaxXP = 15;
 static var PlayerLVL : int;
 static var LVLText = "Level: 0";
 static var MaxLVL : int;
 var Sting : GameObject;
 var Axe : GameObject;
 var Die : AudioClip;
 var LevelUp : AudioClip;
 var other : KillSys;
 var ManaObj : GameObject;
 
 public function KillCounter(){
         audio.PlayOneShot(Die);
         PlayerXP += 8;
         XPText = "XP: " + PlayerXP + "/" + MaxXP;
         if (PlayerXP > MaxXP)
         {
             audio.PlayOneShot(LevelUp);
             PlayerXP = 0;
             PlayerLVL += 1;
             LVLText = "Level: " + PlayerLVL;
             MaxXP += 8;
             UpDamage();
         }
 
             
     }
 
 function OnGUI ()
 {
 GUI.Box (Rect (Screen.width*0.6-51, 30, 102, 22), XPText);
 GUI.Box (Rect (Screen.width*0.7-51, 10, 102, 22), LVLText);
 }
 
 function UpDamage()
 {
     ManaObj.MaxMana += 50;
     Axe.TheDammage += 10;
     Sting.TheDammage += 10;
 }