Exception when calling checkContentChanged on editor which is not in focus

Description

When clicking button, its event handler calls editor.checkContentChanged(). Error is thrown.

Uncaught TypeError: target.getAttribute is not a function
    at Events.updateInput (medium-editor.js:2921)
    at MediumEditor.checkContentChanged (medium-editor.js:7771)
    at HTMLButtonElement.<anonymous> (pen.js:3)

Steps to reproduce

1. Trigger checkContentChanged when target editor is not focused.

Expected behavior: Trigger update check

Actual behavior: Exception is being thrown

Link to an example: https://codepen.io/laacz/pen/JxmjZG

Versions

• medium-editor: 5.23.3
• browser: Chrome win 71.0.3578.98
• OS: Windows 10