Arduino IDE Quits On Launch in MacOS El Capitan

I downloaded the most recent Arduino IDE onto my MacBook Pro running El Capitan, and when the initial blank sketch shows up, the IDE crashes without notice. Do I need a newer version of Mac OS?

Arduino IDE 1.8.8 has a bug that causes it to be incompatible with any macOS version older than 10.12. The bug has been fixed, but there has not been an IDE release since that time. You can use Arduino IDE 1.8.7 until the next Arduino IDE release comes out. You can download Arduino IDE 1.8.7 here:
https://www.arduino.cc/en/Main/OldSoftwareReleases#previous