I did that before installing SA2 and then again after seeing the error, but this didn't change anything.
When I tried to uninstall SA2 so that I could re-install SA1, I started to get more error messages from the extension manager saying that I could not remove it. In the end, I have had to remove Dreamweaver CS4 and re-install it.
As I didn't remove the validation toolkit before installing SA2, do you think this could have caused this JS error? I will try again, this time making sure I remove the toolkit first. Will let you know if this error occurs again.