Drop down menu linking problem!!!

now, i am doing drop down menu which learn form youtube. i try to create the button in the drop down menu. when i click the button to link to other scene, it appear errors. WHY? can anyone help me to solve this problem.

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TypeError: Error #1009: Cannot access a property or method of a null object reference.
        at dropdwonmenu3_fla::MainTimeline/frame64()
        at flash.display::Sprite/constructChildren()
        at flash.display::Sprite()
        at flash.display::MovieClip()
        at dropdwonmenu3_fla::aboutsubmenu_3()
        at flash.display::Sprite/constructChildren()
        at flash.display::Sprite()
        at flash.display::MovieClip()
        at flash.display::MovieClip/gotoAndPlay()
        at dropdwonmenu3_fla::aboutsubmenu_3/fl_ClickToGoToScene_2()

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