Blur editor event fails on source editing mode

[Aliaksei]

Steps to reproduce

1. Download ​http://download.cksource.com/CKEditor/CKEditor/CKEditor%204.6.2/ckeditor_4.6.2_full.zip and unpack
2. Open samples/index.html in Chrome browser
3. Click 'Source' button on toolbar to switch CKEditor to source editing mode
4. Open Chrome Inspector
5. Open Console tab
6. Execute snippet

   CKEDITOR.instances.editor.on('blur', function(e) {
   			console.log('blur happened');
   		});
   

7. Click inside on CKEditor text area
8. Click outside of CKEditor

Expected result

Console should print blur happened

Actual result

ckeditor.js:238 Uncaught TypeError: Cannot read property '$' of undefined
    at CKEDITOR.focusManager.d (ckeditor.js:238)
    at CKEDITOR.focusManager.<anonymous> (ckeditor.js:238)
    at ckeditor.js:28

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

• CKEditor 4.6.2 Full pack (where ​http://ckeditor.com/addon/sourcearea is enabled)
• Chrome 55.0.2883.87 m (64-bit)
• Windows 10

[Jakub Ś]

DUP of #16809