Welcome to the world of beta testing. There are going to be incompatibilities and bugs in any beta product. I hope for your sake you have a way to boot to an approved system. I have no idea what you can do to fix this problem with this build. When I do beta testing in a production environment I always have a way to reboot the system to a configuration that I know works. I usually do this with a backup external hard drive.
I am not getting that bug. I get this on startup though.
After Effects can’t continue: unexpected failure during application startup
Indeed, that is all the error message says, but if you check Console, which is in /Applicaitons/utilities/Console.app, and search for after effects, you should see the line saying: Assertion failure in -[NSMenu itemAtIndex:], /Library/Caches/com.apple.xbs/Sources/AppKit/AppKit-1389/Menus.subproj/NSMenu.m:947