Okay, you've stumped me.
I've registered a sidebar that has only one purpose - to be a replaceable widget container inside of a user-created Parallax section. All told, it works pretty well, unless I don't have the Parallax template selected. See the attached image.
Have you included a way to [a] only show the sidebars metabox if a sidebar is available and [b] make a specific sidebar only available to a specific page template?
I can probably write that functionality myself but I'd rather not. Am I missing an option?