Design Extender has not been updated to work with php7 yet. I'm working on the upgrade to Design Extender now. I have to update menu writer, and I'm doing an overhaul of the page and site sculptor functionality to be responsive and work with more complex designs.
For now you can manually update your pages. I have posted a zip with an upgrade for php7 here: http://www.webassist.com/forums/posts.php?id=40806