Whenever FCP crashes or starts acting strangely an easy thing to try is resetting FCP preferences. Hold Option and Command while launching FCP and choose Delete Preferences. It will open the default Untitled library. Re-open the library you were working on, set the Preferences back to the way you like them (disable background rendering, reset Import to Leave Files in Place, etc.) and see if the issue has been resolved.
If you are still experiencing issues after that, run an etrecheck report from https://etrecheck.com after giving it Full Disk Access in System Preferences. Post the report here using the "Additional text" button below in your reply. This will help us to see what might be going on with your machine. You can search other discussions here to see what other people's reports look like.