plugin:dialog JavaScript error at opening dialog boxes in IE

[Martin Kou]

To reproduce the bug:

1. Open replacebyclass.html in IE6.
2. Triple click to select the "CKEditor Sample" title in the page.
3. Open any dialog - link, image, table, etc.
4. You get a JavaScript error if the dialog tries to restore the selection.
5. Close the dialog by clicking cancel.
6. You get another JavaScript error.

[Garry Yao]

DUP of #2816.