Prefab running once as planned, crashing Unity on second run

I'm trying to instantiate a user-controlled avatar (in this case a simple cube from a prefab).

There must be something wrong with the way my prefab is set up, because my program is working as intended when I drop the necessary scripts onto an existing GameObject cube.

When I run my program once, it works fine with all the scripts triggering as they should. I stop and run again and Unity crashes to desktop.

Here's the code that I think is suspect. Being a novice C# and Unity user, I wouldn't be surprised if I'm doing it wrong, so any help is greatly appreciated.

Thanks in advance.

public class InstantiatePlayer : MonoBehaviour {

public new Transform PlayerBlock;

void Start () { //place movement and behavior scripts onto transform PlayerBlock.gameObject.AddComponent("PlayerMove"); PlayerBlock.gameObject.AddComponent("ColorChangeBehavior"); //put a new moveable cube with behavior script on it at origin point Instantiate(PlayerBlock, new Vector3(0.0f,0.0f,0.0f), Quaternion.identity); }

}