Not sure what caused this. I just started Zotero and while it was initializing it crashed. I'll mention things that may or may not be relevant below.

1. I haven't used Zotero on this computer for well over a month. In fact, I was out of town for a month, and during this time the computer (an iMac) was in the repair shop while its GPU was being replaced.

2. When Zotero started up, so did Better Bib(La)TeX. It reported that cite keys were missing from many of my references and that it was creating them. I went to get a cup of tea.

3. Some time later the first error appeared. It read:

[JavaScript Application]
TypeError: item.getSource is not a function

4. Then, after considerable time while BBT did its work and seemed done, Zotero threw the error that caused me to make this report:

An error has occurred. Please restart Zotero.
You can report this error by selecting "Report Errors ..." from the Help Menu.

5. I then restarted Zotero. Apparently it installed an update during the restart.

6. After Zotero restarted, BBT started too. This time it said nothing about cite keys.

7. The restart seemed stable, so before sitting down to work I went to get a second cup of tea. When I came back the same error as in #3 was showing.

8. This time, clicking the OK button to hide the error message kept on causing the message to reappear.

9. It appears the message has something to do with BBT. Since BBT said it was generating a large number of cite keys (see #2), i.e. > 10K keys, it may take some time before I can try to start using Zotero. If anything significant occurs, I'll come back and append this report.