This is linked to this problem of images not inserting, I think, as it throws the exact same error on the same line.
1. Open a new post or edit a post
2. Switch to the Text tab
3. Switch to the Visual tab
You will see the visual buttons for the TinyMCE under the Text buttons, which stay on the screen, and the Visual button does not appear to be highlighted.
Clicking back to Text removes the Visual button row but there's no way to get to the normal Visual view.
The only plugin active is the Beta Test one, I'm running 3.9-RC1 and the active theme is Twenty Fourteen.
This is in Chrome (latest version) on a Mac.