AlloyEditor throws error when writing on it

• Type: Regression Bug
• Status: Closed
• Resolution: Fixed
• Affects Version/s: Master
• Fix Version/s: 7.0.0 DXP FP18, 7.0.3 CE GA4, 7.0.0 DXP SP4, 7.0.X EE, 7.1.X, Master
• Component/s: Frontend Infrastructure > WYSIWYG
• Labels:
  • liferay-digital-enterprise-70-sp4
  • liferay-fixpack-de-18-7010
• Branch Version/s:
  7.0.x
• Backported to Branch:
  Committed
• Fix Priority: 3
• Browsers:
  Chrome (latest)
• Git Pull Request: https://github.com/brianchandotcom/liferay-portal/pull/49048

Steps to reproduce:

1. Go to Message Boards and create a new Thread.
2. Write something in the "Body" section.

Expected result:
No error appears in console

Actual result:
For each character, this error is shown in the console:

ckeditor.js?browserId=other&languageId=en_US&b=7003&t=1493367812024:304 Uncaught TypeError: Cannot read property 'replace' of undefined
    at new window.CKEDITOR.window.CKEDITOR.dom.CKEDITOR.template (ckeditor.js?browserId=other&languageId=en_US&b=7003&t=1493367812024:304)
    at t._getAriaUpdateTemplate

Note: it also happens in wiki, where in the Title you get a different message error but quite related with this one.