can I set a variable for a component to null

I have a variable that is set to null and/or assigned to a Component during runtime. Is there a way to tell the difference?
I’m getting some crashes on iOS that I’m finding it hard to debug.

I have a script called “jump.js”

var jumpscript:jump;

During the game the script is assigned to the variable or the variable is set to null.
jumpscript=gameObject.GetComponent(jump);//set it
jumpscript=null;//set it to null;

Will either of these tell the difference between these two without crashing?

if(!jumpscript)
or 
if(jumpscript==null)