This is how you can get around it till it gets fixed.
1. When you're ready to export, go to preferences and turn off background rendering.
2. Quick FCXP
3. Go into your event and project that you're working on and delete the 'renders' folders (they'll recreate themselves when you open again).
4. Empty the trash.
5. Re-open FCPX. Export.
That should fix it for now. I think a patch will come soon.