Why have error: NullReferenceException: Object reference not set to an instance of an object

         public class GamePlayManager : MonoBehaviour {
               public void BirdDiedShowPanel(){
                 pausePanel.SetActive (true);
         
                 scoreText.gameObject.SetActive (false);
                 gameOverUI.gameObject.SetActive (true);
         
                 resumeGameButton.onClick.RemoveAllListeners ();
                 resumeGameButton.onClick.AddListener(()=>RestartGameButton());
             }
        }
     
     public class BlueBirdScript : MonoBehaviour {
                void OnCollisionEnter2D(Collision2D target){
                          GamePlayManager t = new GamePlayManager();
                             if (target.gameObject.tag == "Ground" || target.gameObject.tag == "Pipe") {
                  if (birdIsAlive) {
                     birdIsAlive = false;
                     anim.SetTrigger ("Died");
                     audioSource.PlayOneShot (diedClip);
                     GamePlayManager.instance.BirdDiedShowPanel();
                     t.BirdDiedShowPanel ();
                 }
              }
     }
  

why t.BirdDiedShowPanel error (line 22) ???, I don't understand, please explain it