Uncaught TypeError: Cannot read property 'isHidden' of null

Theme: Divi
WP version: 4.0.1
Problem: Cannot edit pages in the admin editor when using the page builder. Instead, the page builder tries to load but just sees a continual spinning circle and it never actually loads
Plugins: Tried disabling all plugins, no change, this isn’t a conflict.
My guy feeling: I have a feeling this may be a compatibility issue with the latest version of WP which has been automatically updated recently

Error listed in developer console; Uncaught TypeError: Cannot read property ‘isHidden’ of null – within admin.js, line 2120

Which is this –> content = typeof window.tinyMCE !== ‘undefined’ && typeof window.tinyMCE.get( textarea_id ) !== ‘undefined’ && ! window.tinyMCE.get( textarea_id ).isHidden()

Theme was customised slightly, nothing major so shouldn’t have cause any problems as this hasn’t been changed in around 6 months. Didn’t use a child theme at the time unfortunately.

After hours of trying to get to the bottom of this, it started working randomly for a brief period, then stopped again. When it does briefly work, it then seems to break when after clicking on the visual editor and it doesn’t load

Hope this explains the very odd problem that I’m seeing. Can’t get my head around what could be causing this. Any advice appreciated