The problem here is that your moveTowardsPlayer script MUST be present on the same game object that this script is attached to due to the reason that GetComponent retrieves a component from ‘this’ gameObject.
If GetComponent does not find a script it returns null, therefore when you do movement.enabled = false; movement does not have a reference therefore giving you the object reference error.
The solution to this is to first retrieve the gameObject that moveTowardsPlayer is present on, and then disable the script.
movement = GameObject.Find("Player").GetComponent<moveTowardsPlayer>();
movement.enabled = false;