Set instantiated player as camera's target

Unity Engine
1

I’m trying to set player, that I instantiate, to Camera_behaviour’s playerPrefab variable so camera could follow it. It gives an error “UnassignedReferenceException: The variable playerPrefab of ‘Camera_behaviour’ has not been assigned. You probably need to assign the playerPrefab variable of the Camera_behaviour script in the inspector.”
I can’t assign it in inspector, because I need to reference the instance of playerPrefab. Anyone could help me how to fix this or give way to get same results by different means in scripting?
Thank you.

Camera_behaviour.js

public var playerPrefab : Transform;

function Update ()
{
transform.position.z = Mathf.Lerp(transform.position.z, playerPrefab.position.z, 0.75); 
transform.position.y = Mathf.Lerp(transform.position.y, playerPrefab.position.y, 0.75); 
}

Player_spawn_script.js

var playerPrefab : Transform;


function Start () 
{
var playerInstance = Instantiate(playerPrefab, transform.position, Quaternion.identity);

var cameraScript : Camera_behaviour = gameObject.GetComponent(Camera_behaviour);
cameraScript.playerPrefab = playerInstance;
}
2

If your player character has a tag such as “Player,” I would throw down something like this in the update for your camera script, where “target” is the variable for your player:

function Update()
{
     if(target == null)     //If for whatever reason, we lose a link to a target, attempt to make a new one.
          target = GameObject.FindWithTag("Player");

    // Rest of your code here.
}