Cancelling 'Move Post' leads to "error"

Backlight 4, 5 & 6 Support
#1

The Cancel button on the (new in 6.3.0) ‘Move Post’
page /backlight/publisher/move_post/id/view/ leads to page with:

{
  "status": "error",
  "message": "The page sent an action that was not available."
}