why child class can access object while parent can't

Question

I have Block class which is the parent and StandardBlock the child , the prefab block have StandardBlock script attached to it, and according to my small knowledge in inheriting I though this would work

public class Block : MonoBehaviour
{
HUD HUDscript;
// Start is called before the first frame update
void Start()
{
    HUDscript = GameObject.FindWithTag("Canvas").GetComponent<HUD>();  
}
public void OnCollisionEnter2D(Collision2D col)
{
    HUDscript.addPoints();
    Destroy(gameObject);
}
}

and child class

public class StandardBlock : Block
 {

      // some code
}

but this is the error that i got

NullReferenceException: Object reference not set to an instance of an object Block.OnCollisionEnter2D (UnityEngine.Collision2D col) (at Assets/Scripts/Gameplay/Block.cs:24)

while writing OnCollisionEnter2D in the child class worked fine

Answer 1

Since I used a start method too in the child class I should have written it that way

override protected void Start()
{
    //some code
    base.Start();
}

and in the parent class

virtual protected void Start()
{
    HUDscript = GameObject.FindWithTag("Canvas").GetComponent<HUD>();


}

so now the child can implement both methods

Thanks to @DMGregory