To reproduce:
1. Open attached project
2. Open scene test
3. Enter play mode. Cubes will start moving into a green square, which destroys them. Along this, a 'OnTriggerStay2d' is being called and Collider2D.attachedRigidbody is being used for setting the velocity of the cubes.

Once this happens to a cube that's been called on Destroy(GameObject) - exceptions get thrown. This does not happen consistently, but once every few cubes.

If I use GetComponent<Rigidbody2D>() in the Conveyor script instead, I get no exceptions.

Explanation: The option of Physics2D 'changeStopsCallbacks' has been turned-off which means that even if an object involved in the callback has been deleted during callback processing then any remaining callbacks should continue.

The exception is caused by accessing the Rigidbody2D which has been deleted. Either turn-on the 'changeStopsCallbacks' or ensure that you check that the Rigidbody2D is not null before accessing its members.