-- "2 buttons.txt" inside the attached zip is actually "2 buttons.fla", please extract and rename.
In the attached fla file, click on "bt1' takes the timeline to frame 2. click on "bt2" returns to frame 1
and thrown the error:
TypeError: Error #1009: Cannot access a property or method of a null object reference.
from this point, bt1 is not functioning anymore.
Now I've found that if bt1 does not contain any other symbol inside it (currently it contains a movie clip), the problem does not appear.