When passing a value it becomes null

I have a script called update, this script is given to another script via:

 public Update up;

This script is not null as I am successfully able to do:

 up.run(1.0f);

I then need to pass up to a script of my player object which i have created through:

 cam.SetTarget((Instantiate(player,Vector3.zero,Quaternion.identity) as GameObject));

the script is then put into a variable via:

 Player_Movement p =  player.GetComponent<Player_Movement>();

and then I try to give the Player_Movement script the Update script via:

 p.setUpdate(up);

which simply runs a function in Player_Movement like so: private Update backgroundUpdate; public void setUpdate(Update script){ backgroundUpdate=script; Debug.Log ("set"); } this function is being run successfully, as seen in the log. The problem is that I am then trying to use the run function of the Update script from the player and then I get a null reference exception. The Update was set before I tried to run this.

I don't have much experience with unity but to me it seems like a really obtuse way to do it, as all I am trying to do is set a script in a prefab to a script possessed by a gameObject, however I could not find any way to do it except through such a convoluted manner. However even if this is the wrong way to do this (I am sure it is) I would like to know why I am getting a null reference exception. I apologize if this isn't clear, please do not hesitate to ask any clarifying questions.