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

Put GamePlayManager on a GameObject in the scene and find it. You cannot create and allocate a new component that is not attached to anything.

//GamePlayManager t = new GamePlayManager();
GamePlayManager t = FindObjectOfType<GamePlayManager>();