TypeError: Error #1009: Cannot access a property or method of a null object reference

i put

MovieClip(parent).more_btn.addEventListener(MouseEvent.CLICK,clickmore); code

inside one movieclip.when i am clicking then more btn is coming but when i overing or clicking the more btn it is giving this error

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at Capgem2_fla::slnDef_1/frame1()[Capgem2_fla.slnDef_1::frame1:81]

plz give some idea to solve this problem .