bug: scrolling generates an error and cancels a running script

I´ve got a scrollView (does not matter if its with a textView or TableView inside) with scrollbars.

My Script is busy with some stuff (a count down), and if i click on an arrow of the scroll bar, I get an error message, and the execution of the script is stopped.

set countDown to 5
repeat until countDown = 0
	delay 1 -- problem exists with or without this line
	set countDown to countDown - 1
	set the content of the text field "countDown" of window "main" to countDown as string
end repeat

The Error in the Log is:

The strange thing is: using the scrollwheel of the mouse does not generate the error, neither does dragging of the scrollbar. But clicking the scroll arrow does.
Clicking on the arrows of a DatePicker also generates the error.

Any help welcome.

here is my website with the Applescript errors (all of them) http://www.geocities.com/eddybaur15/applescript/error.html