Which Lightroom version? I am sure you have also tried rebooting your machine? Try holding option and shift while starting Lightroom and resetting its preferences.
The version is 2015.10
Thanks for your response. I have rebooted, as you can imagine, and reset preferences. If this is any help, here is an excerpt from the crash report produced by Apple:
Process: Adobe Lightroom 
Path: /Applications/Adobe Lightroom/Adobe Lightroom.app/Contents/MacOS/Adobe Lightroom
Version: Adobe Lightroom  (6.10)
Code Type: X86-64 (Native)
Parent Process: ??? 
Responsible: Adobe Lightroom 
User ID: 501
Date/Time: 2017-05-21 12:07:58.487 -0700
OS Version: Mac OS X 10.12.5 (16F73)
Report Version: 12
Anonymous UUID: 108CFE34-E44F-8D74-2841-73BEB4C5C5C1
Sleep/Wake UUID: F2ECC210-60AA-4833-9BA4-A3B22AB4CE60
Time Awake Since Boot: 2500 seconds
Time Since Wake: 280 seconds
System Integrity Protection: enabled
Crashed Thread: 17 Worker Thread
Exception Type: EXC_BAD_ACCESS (SIGSEGV)
Exception Codes: KERN_INVALID_ADDRESS at 0x0000000000000000
Exception Note: EXC_CORPSE_NOTIFY
Termination Signal: Segmentation fault: 11
Termination Reason: Namespace SIGNAL, Code 0xb
Terminating Process: exc handler 
Thanks for that info. This is clearly a real crash of Lightroom. It is incredibly hard to troubleshoot this unfortunately. The crash report (this is only part of it by the way) shows that Lightroom for some reason tries to access a part of memory that it shouldn't or that doesn't exist. That can be a real bug, some problem with your installation(probably not as you reinstalled) or some other software that is injecting itself. Do you have anything like an antivirus, cleanup utility, little snitch or anything other third party system utility installed?
I do have an antivirus installed. I shut it off, rebooted, deleted Lightroom, signed out of Creative Cloud, rebooted, signed back in, reinstalled Lightroom. Same problem. It's a mystery.
Thanks for you suggestions. I'm not sure what to do next.
I spent time with Adobe, and we resolved. It was determined that another version of LR had been issued (6.1). So the problem, is that I couldn’t get into Lightroom to update my version. Also LR sends update notices in email, but doesn’t auto update or send notices within LR, except if you look for it. A pathetic process, which says they don’t care about LR owners…as they want you to go with the monthly version (Creative Cloud). This was quite shocking, and undermines my trust of Adobe. I would get CC, but it’s basically a commitment to pay/use it for a LIFETIME, or it will stop. That is super-crazy.