Cannot undo after removing uploading image.

[kkrzton]

Steps to reproduce

1. Go to ​http://tests.ckeditor.dev:1030/tests/plugins/uploadwidget/manual/image.
2. Drag and drop 3 images to the editor.
3. Delete 2 of them while images are still uploading.
4. Undo.

Expected result

It is possible to undo changes.

Actual result

Undo does nothing. Error in the console is thrown: Uncaught TypeError: Cannot read property 'data' of null(…).

Other details (browser, OS, CKEditor version, installed plugins)

Checked on Opera and Firefox. Occurs in both browsers.