After I have deleted a one or more photos, I sometimes get an error message saying:
"Error: An internal error has occurred: ?:0: attempt to index a nil value."
What's this all about? And what can I do to stop it from coming up?
That is an Adobe programming error related to the catalog (database).
Your best bet is to write a bug report at
Photoshop Family Customer Community
using all the detail you can.
Retrieving data ...