Thanks for the detailled report, I provided a fix. Incidentally, it is similar to another fix, for the exact same problem I submitted today for the content pane.
That said, you can workaround it by adding the ibm.xsp.widget.layout.xspCKEditor module at the page level. I would advise to do so even with the issue fixed, as it allows the module to be aggregated with the other JS files. Else, it will be loaded later as a regular dojo module.