Testing buttons to get to other scenes. 1021 error.

[raee]

I changed the instance names to blue and green btn and made sure they were all right in the code but I still get the error.
This is what it says.

TypeError: Error #1009: Cannot access a property or method of a null object reference.
	at test_fla::MainTimeline/frame25()
	at flash.display::MovieClip/gotoAndPlay()
	at test_fla::MainTimeline/mouseDownHandler0()

But whatever. I give up.
Thank you fro trying.