Steps to reproduce

1. Go to PlatformUI;

2. Create a content type having a RichText field;

3. Back to "Content Structure", create a content of the content type you just created with Rich Text field;

4. Open Firebug or similar tool and go to Console;

5. Click once on the rich text field to start editing it, and press delete once, and take notice you get an error on the console:

TypeError: block is null                   focusblock.js (line 26, col 8)
block.addClass(FOCUSED_CLASS);

Afterwards, if you try to add any other rich text type onto the field, you won't be able to see its outline and they won't function correctly.