is_rtl() does not work in Plugin with own Themes

I develop a Plugin AppFIX. AppFIX is a PLugin where you can use two themes at the same time. If you put domain.de it show Theme A and if you put domain.de/?system=app it show Theme B. And if i install the Plugin the Plugin is working but he can’t find the function is_rtl() and i get a white screen in the backend.

It isn’t a WordPress Core Bug: https://core.trac.www.ads-software.com/ticket/34208