I've just investigated the problem a bit further more. I use tabcontainer which is an ajax control toolkit. If the editor is not in the tabpanel which is already open (shown) and I don't ever click on the tabpanel which has the editor, I just click on save button which to call a ajax post, and then it will report the js error.
however, if I ever click on the tabpanel which has the editor or make this tabpanel as the default opened tab, I will not get the js error.
I think the problem is, somehow, the editor is not fully initialised if the tabpanel is invisible.
Therefore, is that any way to fully load the editor evenif it's in a invisible tabpanel which can be shown by js?
thx