An internal error has occurred: assertion failed!

Thank you for your prompt response Victoria.    That popup was set to "Do Nothing" but I found the problem:

I kept checking similar questions after I posted and found someone who'd had the same trouble but only on one user account.  This reminded me that Lr had done odd things with permissions on Presets folders in the past which had prevented me from saving new presets.

The backstory:  When I created the main user account on this machine - by restoring from a TimeMachine backup - it somehow gave me a short name with a space in it.  No idea how, but it hasn't caused me any problems in 4 years - except with Lightroom!  Lr sets the ownership on all its folders to the first word of my short user name, which is not me as far as the system is concerned so I don't get RW access.

The solution in this case was to make myself the owner of, and give myself RW access to, the folder ~/Library/Application Support/Adobe/Lightroom/ and everything in it.

I guess Lr does not expect a short name with a space in it, which should be a reasonable assumption given that I don't think you can create them in the OS X gui, and fails to deal with it when it finds one by only parsing the first word.