NullPointerException in ckeditor widget after ajax update

The form content mask contains a ckeditor. The mask will be replaces by a new one containing a new instance of ckeditor (forward navigation in our application). If we navigate back and typing changes in first ckeditor instance the console shows NPE.

![ckeditor_npe](https://f.cloud.github.com/assets/3490002/130986/8da32b38-7044-11e2-970b-2b8f8fcee02c.png)