I'm guessing that the changeTa function creates more textarea-s and adds them to the page.
The problem as I see it is that var bcv has document.getElementById for all possible textareas whether or not they are there.
If you can remove those out of that, you could test to see if they exist, then either assign their values or an empty string to variables to be used by bcv
For example, if there were no textareas, the URL would end up being
which isn't the most efficient and has a lot of unused GET vars in it, but it would work OK
BTW it's good to see you're using & instead of only &
You wouldn't believe how many so called professional apps do it the wrong way.