Inserting list inside widget focused empty editable area causes error

[Jakub Ś]

Steps to reproduce

1. Copy/Paste attached file into your ckeditor folder and open it in a browser (simplebox plugin is a requirement)
2. Insert simple box.
3. Click inside Content... and press Ctrl+A/Del to clear all text.
4. On element's path click on right most div or p.
5. Click Insert List button

Expected result

List gets inserted

Actual result

Java Script error gets thrown
Message: Cannot read property 'isContextFor' of null
Line: 75
Code: /ckeditor-dev/core/command.js

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

Problem can be reproduced from CKEditor 4.3 in Blink browsers only (Safari on Mac works fine)

[Jakub Ś]

Other ticket where same error message gets thrown: #10127, #10559

I'm attaching also a video showing how to reproduce the issue.