I further searched for a solution and found the following similar problem reported by someone else:FCPX crash on file import
It mentioned about 'forcing' the FCPX Import screen to pop up by inserting an SD card (with video files). I tried and it worked... but then failed again if I took the normal click for the "Import" function.
It also mentioned about testing with login via another account. I tried it with the Guest account. The problem did not happen. I then went back to my normal account. It failed on first attempt... but then 'miracle' came when I restarted the computer and tried again... it works. Wow...
Seems the FCPX Import Crashing problem is getting away from me (which is very good)... but then another problem comes up... FCPX (actually my iMac) failed to recognise my 2 external hard disks ( a 2big 6TB Lacie and a Seagate 4TB). Sometimes it can recognise both and sometimes either one and sometimes none... different outcome from different boot up. And I have to manually 'mount' using the Disk Utility. This is very annoying. I tried various fixes but in vain ( eg disconnect hard disk, turn on iMac again, connect hard disk, then turn on hard disk... it mounted... restart iMac... connection dropped again... restart, failed to connect ... restart, connection ok ... restart, failed to connect ... etc etc). And from the web, I saw many others also complaining about the external hard disk connection issue... very annoying. ( By the way, may be I need to post this again with another topic title or join the other topic forums. )glen
Anyway, thanks Allen for the kind advice... but then seems the High Sierra still have quite some instability requiring fixes...