Ball not releasing after 1st goal?

So I’ve been making my first game, Pong, and i’ve run into an error. When the EnemyAI scores a point against me, and the ball respawns, I’m unable to click the ball to start that round. Here’s the code for the ball:

public class Ball : MonoBehaviour
{
public float ballVelocity = 3000;

Rigidbody rb;
bool isPlay;
int randInt;

void Awake()
{
	rb = GetComponent<Rigidbody>();
	randInt = Random.Range(1,3);
}

void Update()
{
	if (Input.GetMouseButton(0) && isPlay == false)
	{
		transform.parent = null;
		isPlay = true;
		rb.isKinematic = false;
		if (randInt == 1)
		{
			rb.AddForce(new Vector3(ballVelocity, ballVelocity, 0));
		}
		if (randInt == 2)
		{
			rb.AddForce(new Vector3(-ballVelocity, -ballVelocity, 0));
	
		}
	}
}

}

Then I get an error message MissingReferenceException: The object of type ‘GameObject’ has been destroyed but you are still trying to access it.
Please someone help me as I have not much experience with Coding.

Anyone???