I restarted Final Cut Pro, tried to Export, and it crashed a second time.
If you're wondering why I wasn't using Compressor; well the truth is that though I think Compressor does a nicer job, it seems to take twice as long to compress the same file, and since YouTube munges everything anyway, I find it quicker and faster to use the Export with QuickTime Conversion option.
After the second crash I was faced with a puzzle. But, I hadn't restarted my MacBook Pro in an age, and I also hadn't installed the ProKit 5.1 update, which is supposed to:
Resolved memory leaks for improved performance.So I thought that might be the solution.
I installed the update, along with a bunch of other things that Software Update found, including a Firmware update for the SuperDrive. Unfortunately, this required multiple restarts. All in all, I spend at least half an hour getting everything up to date.
With that all done, I launched Final Cut Pro, opened the project, started exporting....and it crashed in the same place!
But I was not defeated! I just exported to a QuickTime file (instead of with conversion) and then used QuickTime Player to export from that file to an MP4 file.
All in all, it probably would have taken just as long if I'd used Compressor!