I had a crash problem also. I totally uninstalled FCPX using apples uninstall directions and re-downloaded from the app store. After reinstallling, it has worked great.
A couple of things spring to mind
1. Run disk utility over your system, always a worthwhile step after an update, Repair premissions and disk.
2. Delete you render file entirely and start your render from scratch, either use FCPX to delete the render files or do so in the finder.
let us know how you go.