Object reference not set to an instance of an object (Raycast)

I have just finished a game for windows phone, and i have had this error all thorough the process and it has never stopped me doing anything. However when I export to visual studio it will not run because of this error.

Error : NullReferenceException: Object reference not set to an instance of an object GameManager.Update () (at Assets/Scripts/GameManager.cs:90)

 void Update () {
     
     RaycastHit2D hit = Physics2D.Raycast(Camera.main.ScreenToWorldPoint(Input.mousePosition), Vector2.zero);
 
     if(Input.GetMouseButtonDown(0)){
             mouseClicked = true;    
                                
         //Clicked Good Box 1
             if(hit.collider.tag == "GoodBoxTag1"){
             Debug.Log ("GB1 Hit");
             
             //Timer Starts
             //GB1Hit = true;    
             GB1Clicked = true;
         }
       }
   }

The error occurs with (hit.collider.tag == "GoodBoxTag1") I have seen other similar problems and the fixes but they all do a different method of ray casting. Any help with this would be greatly apprenticed. (P.S. This is my first post so sorry for any incorrect formatting)