Easiest way to fix this is to replace the is not visible by system else.
This is actually the best example for a place to use the system else event.
Another way, of which I'm fond myself to make the toggle is:
on button touched : system set timescale : 1-timescale
when timescale = 1, 1-1 returns 0
when timescale = 0, 1-0 returns 1