Cannot read property 'blockLimit' of null when clicking combo box

[Tade0]

Steps to reproduce

1. Consider this jsFiddle: ​https://jsfiddle.net/12o9kyr6/
2. Switch to source mode.

3 Delete all contents.

4. Switch back to WYSIWYG mode, but don't focus the editor.
5. Click any of the combo boxes.

Expected result

Combo box is opened, no error thrown.

Actual result

Combo box is opened, but an error is thrown:

Uncaught TypeError: Cannot read property 'blockLimit' of null
    at CKEDITOR.style.checkApplicable (ckeditor.js:484)
    at $.onOpen (ckeditor.js:980)
    at $.e.onShow (ckeditor.js:787)
    at $.showBlock (ckeditor.js:692)
    at Object.execute (ckeditor.js:784)
    at $.<anonymous> (ckeditor.js:784)
    at ckeditor.js:31
    at Object.callFunction (ckeditor.js:31)
    at HTMLAnchorElement.onclick ((index):1)

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

[Marek Lewandowski]

Removing milestone, it's not a priority ATM.

[Jakub Ś]

DUP of #12326.