Object collision triggering null reference at runtime when colliding with clone of itself after being childed.

I have a rigidbody2D on a ball object that is moving toward a center object. If it hits the center object it successfully stops moving and childs itself to that object and moves with it.
That collider on that ball is now a compound collider (I think) of the center object (its parent), so any other balls that hit that ball should also trigger collision and become part of the object. However, balls don’t seem to trigger collision every time with each other, but they 100% work on the center object. The balls physically stop moving and touch, so they’re clearly colliding and physics is working, but the trigger method isn’t being called, and I don’t know why.

In the Ball Class:

void Update() {
   if (!attached)
      transform.position = Vector3.MoveTowards (transform.position, origin, velocity);
}
void OnTriggerEnter2D() {
		attached = true;
		transform.parent = GameObject.Find ("Triangle").transform;

	}

Dammit, I fixed everything by just doing a null check before setting newly instantiated balls to a parent.

void OnCollisionEnter2D(Collision2D c) {
		attached = true;
		transform.rigidbody2D.isKinematic = true;
		if(this.transform != null && c.transform.parent != null)
			this.transform.parent = c.transform.parent.transform;
	}