We had the exact same problem; rename/delete/move the wpstudio-testimonial-slider plugin folder and everything’s back up again.
Experienced a similar problem: blank back-end editor screens in both pages and posts. Also, no visual editor, just text.
Two observations: 1) it’s possible to select the code, which then becomes visible and 2) the front end is not affected in any way.
What worked was disabling the CKEditor for WordPress plugin.
Hope this helps.
