Null reference exception

 public class paint : MonoBehaviour {   
     public float amtToMove;
  void Update () {
         amtToMove=Time.deltaTime*10;    
         transform.Translate(-Vector3.forward * amtToMove);
 }
 }
 In my paint class i have the behaviour of the paint object. 
    
 public class car : MonoBehaviour {
 
     public GameObject paint;
     public Transform paintPrefav;
  void Start () {
         paint = GameObject.FindGameObjectWithTag("paint");
  }
  
  void Update () {
         if (Input.GetMouseButtonDown(0)) 
         {
             Instantiate(paintPrefav, new Vector3(-5.3f, 26.7f, 20.97f), paint.transform.rotation);
         }

In the class car, I instantiate the prefab. Drag and drop on the unity interface is done. After all this, i am getting Null reference exception on the line where I instantiate the Prefab. Please tell me where am i going wrong. Thanks in advance.