Looks good now. It crashed once after about 80,000 blocks, but afterwards sync'd up fine overnight. I removed almost all the Dash to another wallet, er, just in case. Thanks very much guys for the rapid help.
Yes, there was an update, which I installed a few days ago. But DashCore had already been crashing for a week or so before that. The update made no difference to the crashing.
Thanks for the suggestion. I made a copy of wallet.dat, then deleted all the .dat files except wallet.dat. I then opened DashCore and the same thing happened, namely it started to load the block index then crashed after a minute.
I rebooted, same result.
I've been running Dash Core for 6 months or more on my Mac mini (OSX 10.13.4 High Sierra), no problems. A couple of weeks ago it started crashing within a minute of starting it up. There was no obvious change just before it started to fail. I have some Dash in the wallet, 5 maybe. I updated to...