actionscript 3 - removing instance after excecuted 2 -

got parameter child must non-null error code can ?

for (i=1; i<=4; i++) {     this["box" + i].addeventlistener(event.enter_frame, move);     this["box" + i].addeventlistener(mouseevent.click, action); } function move(e:event):void {     e.target.y -=  6;     if (e.target.y <= 56)     {         movieclip(e.target).removeeventlistener(event.enter_frame, move);         removechild(getchildbyname(e.target.name));         comment.text = "miss";         miss +=  1;         score +=  0;     } } function action(event:mouseevent):void {     if ((event.target.hittestobject(bts1))&&(event.target.hittestobject(bts2))&&(event.target.hittestobject(bts3))&&(event.target.hittestobject(bts4)))     {         movieclip(event.target).removeeventlistener(mouseevent.click, action);         removechild(getchildbyname(event.target.name));         comment.text = "perfect";         perfect +=  1;         score +=  10000;     }     else if ((event.target.hittestobject(bts2)&&event.target.hittestobject(bts3)&&event.target.hittestobject(bts4))||(event.target.hittestobject(bts1)&&event.target.hittestobject(bts2)&&event.target.hittestobject(bts3)))     {         movieclip(event.target).removeeventlistener(mouseevent.click, action);         removechild(getchildbyname(event.target.name));         comment.text = "great";         great +=  1;         score +=  5000;     }     else if ((event.target.hittestobject(bts3)&&event.target.hittestobject(bts4))||(event.target.hittestobject(bts1)&&event.target.hittestobject(bts2)))     {         movieclip(event.target).removeeventlistener(mouseevent.click, action);         removechild(getchildbyname(event.target.name));         comment.text = "good";         +=  1;         score +=  2500;     }     else if ((event.target.hittestobject(bts4))||(event.target.hittestobject(bts1)))     {         movieclip(event.target).removeeventlistener(mouseevent.click, action);         removechild(getchildbyname(event.target.name));         comment.text = "boo";         boo +=  1;         score +=  1000;     }     else     {         movieclip(event.target).removeeventlistener(mouseevent.click, action);         removechild(getchildbyname(event.target.name));         comment.text = "miss";         miss +=  1;         score +=  0;     }  } 

btw "bts1,2,3" line movie clip problem occur on mouse click event, want when clik box hittest on "bts" , box remove after user click, if user didn't click disappear @ point

before removing child , must remove listeners attached , enter_frame , click etc . copy , paste same lines (it's little bit bad practice , when fixed you'll need make refactoring here).