null reference exception help

I made spikes model and imported in unity and added Mesh Collider( convex + Is trigger), and made script if the player gets in collision with the spikes the level gets restarted. But i get this error nullreferenceexception object reference not set to an instance of an object.
Here is my code:

#pragma strict

var gameMaster : GameMaster;

function OnTriggerEnter (colInfo : Collider) 
{
	if(gameMaster.isRestarting == false)
	{
		Debug.Log("Test");
	    if(colInfo.tag == "Player") 
	    {
	        var destructible : Destructible = colInfo.GetComponent("Destructible") as Destructible; 
	        destructible.Destruct(); 
	    }
	gameMaster.RestartLevel();
	}
}

how can i fix this?

Are you defining which instance of GameMaster you want to use? In the inspector are you dragging a GameMaster object into the field for your local gameMaster? If not, then you need to set gameMaster equal to something in the code.