An error I don't know how to fix

Hi, I got CS4 for Cristmas, and I am having trouble with my buttons. I have a button on each frame programmed to go to the next frame when I click it. All of the buttons worked fine until I put a movie clip on frame 12. Now when I click to get to frame 13, it goes back to frame 7. This is what it says in the output:

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at WelcometomyChristmas_fla::MainTimeline/frame12()
at flash.display::MovieClip/gotoAndStop()
at WelcometomyChristmas_fla::MainTimeline/mouseDownHandler10()
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at WelcometomyChristmas_fla::MainTimeline/frame2()
at flash.display::MovieClip/gotoAndStop()
at WelcometomyChristmas_fla::MainTimeline/mouseDownHandler()
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at WelcometomyChristmas_fla::MainTimeline/frame3()
at flash.display::MovieClip/gotoAndStop()
at WelcometomyChristmas_fla::MainTimeline/mouseDownHandler1()
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at WelcometomyChristmas_fla::MainTimeline/frame4()
at flash.display::MovieClip/gotoAndStop()
at WelcometomyChristmas_fla::MainTimeline/mouseDownHandler2()
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at WelcometomyChristmas_fla::MainTimeline/frame5()
at flash.display::MovieClip/gotoAndStop()
at WelcometomyChristmas_fla::MainTimeline/mouseDownHandler3()
TypeError: Error #1009: Cannot access a property or method of a null object reference.
at WelcometomyChristmas_fla::MainTimeline/frame6()
at flash.display::MovieClip/gotoAndStop()
at WelcometomyChristmas_fla::MainTimeline/mouseDownHandler4()

Can anyone help me to fix it please? I don’t have a clue what any of it means. Thanks