WYSIWYG Field not swapping between editor modes

I have added the WYSIWYG editor on a custom post type but I can't toggle between the visual and plain text editors.

I'm getting the following error in my dev console:

TypeError: j is undefined[Learn More] 3 load-scripts.php:702:681
e http://localhost/wp-admin/load-scripts.php:702:681
c/< http://localhost/wp-admin/load-scripts.php:702:271
C http://localhost/wp-includes/js/tinymce/wp-tinymce.php:3:11105
d http://localhost/wp-includes/js/tinymce/wp-tinymce.php:3:11256

My code for the metaboxes is:

public function generateMetaBox($meta_boxes){
    $prefix = strtolower($this->attributes['post']['single']);
    // Content Area
    $meta_boxes[]=array(
        'id'=>'content',
        'title'=>'Case Study Content',
        'post_types' => $this->attributes['post']['slug'],
        'context'    => 'normal',
        'priority'   => 'high',
        'fields' => array(
            array(
                'name'          =>  'cs-content',
                'desc'          =>  'THIS NEEDS SOME INSTRUCTIONAL COPY',
                'id'            =>  $prefix . '_content_group',
                'type'          =>  'group',
                'clone'         =>  true,
                'sort_clone'    =>  true,
                'fields'        =>  array(
                    array(
                'name'  => 'Content Block',
                'desc'  => 'Content block test',
                'id'    => $prefix . '_content',
                'type'  => 'wysiwyg',
            )

                )
            ),
        )
    );

    return $meta_boxes;
}

There are no other plugins installed and the CPT doesn't have the standard editor.

What am I doing wrong here?