Undo of modified footnote not working

[trebaxino]

Hi, i'm using your plugin and few days ago i realized that undo functionality for the newly added and modified footnote not working.
Steps to reproduce:

1. Open demo of CKEditor with footnote plugin using link in your readme.
2. Add new footnote to the text
3. Modify footnote content
4. Press undo and exception appear.

Uncaught TypeError: Cannot read property 'filter' of undefined at n (ckeditor.js:800) at a.<anonymous> (ckeditor.js:808) at a.m (ckeditor.js:10) at a.<anonymous> (ckeditor.js:12) at a.CKEDITOR.editor.CKEDITOR.editor.fire (ckeditor.js:13) at a (ckeditor.js:368) at a.CKEDITOR.editor.selectionChange (ckeditor.js:385) at $.<anonymous> (ckeditor.js:382) at $.m (ckeditor.js:10) at $.fire (ckeditor.js:12)

[andykirk]

Hi,
Thanks for reporting.
I'll look into this when I can, but I have very little time to spend on this at the moment.