Steps to reproduce the issue

Install Joomal 4.0-dev @ 1f24603
Login to super admin
Click Install next to Blog Sample Data
Click ok in JS popup

Go to front end and

Login as super admin

click Welcome to your blog
Click cog -> click edit (to edit the article)

Click "CMS Content" button in editor
Select Image
DONT SELECT AN IMAGE
click "Insert Image:

Expected result

No console errors

Actual result

(Ignore the first two errors in screenshot - that's TinyMCE and Safari upstream issue)

Unhandled Promise Rejection: Error: Nothing selected

System information (as much as possible)

safari on Mac