IE9: deleting iframe in BR mode causes JS error

[Jakub Ś]

How to reproduce:

1. Open enter key sample and switch to BR mode.
2. Use the developer console to set the editor data

   CKEDITOR.instances.editor1.setData('<iframe></iframe>');
   

3. Set the cursor after the iframe and press enter to make a newline
4. Press backspace to delete an iframe

JS error is thrown:
Message: Can't get property "remove": object is empty or undefined
Line: 171
URI: /ckeditor/plugins/wysiwygarea/plugin.js

NOTE: IE8 has its own problems defined here #8446. Problem can be reproduced from CKEditor 4.0. In CKEditor 4 beta inserting iframe caused another error - insufficient privileges.