Instantiate 2 different Buildings in RTS doesn't work // Null Reference Exception

Hey Guys,

maybe its a real easy problem, but i'm a very beginner with Unity :)

I'm building a RTS Game, and actually i have a little map, and a GUI with 2 Icons for 2 seperate buildings. Now when i click one of the GUIs it instatiates the building and yeah, i can build it. But when i click on the other GUI afterwards, it instaites the other building but moves the first instatiation with the actual one. So i'm moving the new building and the old one i just built. Error is: NullReferenceException: Object reference not set to an instance of an object.

The Code of both GUIs:

var building : Transform;
var gridSpacing = 2;
var instant;
var ray;
var hit:RaycastHit;

function OnMouseDown() {

    if(buildingCounts.at_Buildt == true)
    {
        /* var p : Vector3 = Camera.main.ScreenToWorldPoint (Vector3 (Input.mousePosition.x, Input.mousePosition.y, 30));
        Instantiation = Instantiate(building, p, Quaternion.identity);
        */
        ray = Camera.main.ScreenPointToRay (Input.mousePosition);

        if (Physics.Raycast (ray, hit, 100)) 
        {
            Debug.DrawLine (ray.origin, hit.point);
            instant = "";
            instant = Instantiate(building, Vector3(hit.point.x, 2.5, hit.point.z),Quaternion.identity);
            instant.name = "Building2_"+(buildingCounts.BUILDING2+1);

        }

        buildingCounts.at_Buildt = false;
    }

}

function Update() {

    if(buildingCounts.at_Buildt == false)
    {
        //var p : Vector3 = Camera.main.ScreenToWorldPoint(Vector3(Input.mousePosition.x, Input.mousePosition.y - 30, 30));
        //Instantiation.transform.position = p;

        ray = Camera.main.ScreenPointToRay (Input.mousePosition);

        if (Physics.Raycast (ray, hit, 100)) 
        {
            Debug.DrawLine (ray.origin, hit.point);
            instant.position.x = hit.point.x;
            instant.position.z = hit.point.z;
            instant.position.y = 2.5;

            instant.position.x = Mathf.Round(instant.position.x / gridSpacing) * gridSpacing;
            instant.position.z = Mathf.Round(instant.position.z / gridSpacing) * gridSpacing;

        }
    }

}

Would be thankful for every help!

ok problem solved, stupid mistake