[D&D] Image into nested editable throws error

[Szymon Cofalik]

Browser: Opera, maybe others.

Steps to reproduce:

1. Create widget image with caption using image2 plugin.
2. Create an inline image outside widget image.
3. Try to D&D the inline image into the widget image.

Expected result: Inline image is no longer at previous position but is now in the caption.

Result: Error thrown
Cannot read property 'getAscendant' of null

Note: It works fine if you select some text beside inline image and try to D&D the text with the image. Note: I am able to cut the inline image and paste it into caption, so it is D&D bug.

[Jakub Ś]

Problem can be reproduced in all browsers from CKEditor 4.5.0 (Occurs also in 4.5.7 and in current master)