How to solve Error#1009?

i getting getting error #1009 when add if loop working code.  prior lines added worked fine.  can please tell me why getting error , how avoid it?  here error message , relevant code:

 

typeerror: error #1009: cannot access property or method of null object reference.

@ piedacoulisse_hits_fla::maintimeline/reset()
@ piedacoulisse_hits_fla::maintimeline/blockit()

 

 

function reset(event:event):void{
if(contactpoint == "top"){
if(startmovex > event.currenttarget.x){       // problem line
   body_mc.x = body_mc.x - 2;                    
  }else if(startmovey>event.currenttarget.y){ // problem line
   body_mc.y = body_mc.y - 2;                       // problem line
  }                                                                  // problem line
}else if(contactpoint == "bottom"){
  slider_mc.x = slider_mc.x + 2;
}else if(contactpoint == "body"){
  body_mc.y = body_mc.y - 2;
}
}

 

function blockit(event:event):void{
if(topmask.hittestobject(phone_mc) == true){
  reply.text = "top hit";
  event.currenttarget.stopdrag();
  contactpoint =  "top";
  reset(null);

}

possibly not getting body_mc movieclip put trace before it

and see getting.

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