Removing paragraph below nested widget and moving the nested widget removes main widget editable

[Jakub Ś]

Steps to reproduce

1. Use the attached simplebox1 widget (the same as simple box but content editable allows everything)
2. Clear editor contents and insert simplebox1
3. Select Content... and insert another simplebox1
4. At this stage you can see Paragraph below nested simplebox1 (#12778)
5. Click on that paragraph, next click on P in elements path and press Backspace to remove this paragraph
6. Move nested outside of the master widget

Expected result

The content editable of master widget stays

Actual result

The content editable of master widget is removed

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

Problem can be reproduced in all browsers.