如何修复AS3 TypeError：错误＃1009：无法访问空对象引用的属性或方法？

I am getting this error:

我收到此错误:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at Skool_fla::MainTimeline/frame1()[Skool_fla.MainTimeline::frame1:10]
    at flash.display::MovieClip/gotoAndStop()
    at Skool_fla::MainTimeline/goback()[Skool_fla.MainTimeline::frame2:22]
TypeError: