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)